Spiral cleavage - metazoa, Biology

Assignment Help:

Spiral Cleavage - Metazoa

In Spiral Cleavage, however, the third and fourth cleavage planes are oblique to the polar axis and the resulting blastomeres do not lie on top of one another but above the furrows between the cells. The spindles during the third cleavage are arranged in the form of a spiral, therefore, the name spiral cleavage. This type of cleavage is seen in all invertebrate except the echinoderms (i.e. in annelids, molluscs, arthropods nemertenes and polyclad planarians).

159_Spiral Cleavage - Metazoa.png

                                              Figure: Spiral cleavage showing transition from 4 to 8 to 16 cell stage

Spirally cleaving embryos are said to have mosaic or determinate form of development. This means that the organ forming regions of the egg are strictly localised in the egg from the very beginning and the fate of the blastomeres is determined early. If the blastomeres are separated, each will continue to develop up to a certain time as though it was a part of the whole and give rise to defective, partial embryos. In the early embryos an unidentified cytoplasmic factor is segregated into one of the blastomeres, the mesentoblast (this is also called the '4 d' cell) which gives rise to the future mesoderm.


Related Discussions:- Spiral cleavage - metazoa

Improvement of cereal proteins - mutual supplementation, Define Improvement...

Define Improvement of cereal proteins - Mutual supplementation? Improvement of cereal proteins: Cereals, in general, are limiting in lysine and threonine while legumes, milk, m

Define mobilization of bone calcium and phosphorous, Define Mobilization of...

Define Mobilization of bone calcium and phosphorous? It is now firmly established that vitamin D 3 is metabloized first in the liver to 25-hydroxyvitamin D (25 OH- D) (calcicl

Explain the ebb-phase of stress response, Explain the Ebb-Phase of Stress R...

Explain the Ebb-Phase of Stress Response? This occurs immediately following injury and lasts for approximately 24-hours. The so-called ebb phase, which is the early phase of th

Explain cephalization in nematodes and platyhelminthes, How is the nervous ...

How is the nervous system characterized in beings of the phylum Annelida? How can one compare cephalization in annelids to cephalization in nematodes and platyhelminthes? Annel

State in detail about the thylakoid membrane, State in detail about the thy...

State in detail about the thylakoid membrane The thylakoid membrane contains all the factors associated with the photosynthetic electron transport leading to the generation of

Chlamydiosis-epidemiology, Epidemiology The organism does not appear t...

Epidemiology The organism does not appear to be very host or tissue specific and can infect naturally a large number of avian and mammalian species. Chlamydiosis has been reco

Brackish water-marine water-types of water found on earth, Brackish Water ...

Brackish Water The content of dissolved salts in brackish water is higher than the fresh water and ranges between 0.5 to 35%. These waters of intermediate salinity range are d

What are the main types of cancer that affect humans, Q. What are the main ...

Q. What are the main types of cancer that affect humans? An Excluding skin cancer, that are the more easily detected and so cases are registered in larger number, the main type

Chromosomes, Chromosomes Karl Nageli and Hofmeister observed chr...

Chromosomes Karl Nageli and Hofmeister observed chromosomes in pollen mother cells of Tradescantia. Chromosome name proposed by Waldeyer. During cell division,

Write Your Message!

Captcha
Free Assignment Quote

Assured A++ Grade

Get guaranteed satisfaction & time on delivery in every assignment order you paid with us! We ensure premium quality solution document along with free turntin report!

All rights reserved! Copyrights ©2019-2020 ExpertsMind IT Educational Pvt Ltd