128 



FUNDAMENTALS OF CYTOLOGY 



94, B). From this stage onward the macromeres divide at a slower rate 

 than the micromeres. Invagination to form the gastrula here begins at 

 the side of the blastula where the regions of large and small cells meet. 



The eggs of birds, reptiles, squids, and bony fish are very strongly 

 telolecithal, the yolk being very densely packed throughout most of the 

 cell but absent from the small germinal disc at the animal pole. The 

 cleavage divisions are restricted to this relatively thin layer of protoplasm 

 and do not extend through the bulk of the cell (Fig. 94, C). After a few 

 divisions have occurred, the 3^oung embryo has the form of a plate of cells, 

 the blastoderm, lying against a large yolk mass, the distinctness of the 



Fig. 94. — Three types of cleavage in animal egg.s. Sections of eggs in early cleavage stages 

 above; surface views of later stages below. Explanation in text. 



boundary between the two regions showing some variation. This is 

 called meroblastic cleavage, in contrast to the holoblastic tj^je which 

 extends through the whole egg. 



A further type of cleavage is observed among insects (Fig. 88). The 

 first few mitoses in the fertilized egg are not accompanied by cytokinesis, 

 the embrj'o being coenocytic during its earliest stages. Soon after the 

 primordial germ cell has been set apart, the multiplying nuclei in the 

 somatic portion of the embryo move to the periphery along with small 

 masses of cytoplasm, leaving the central region holding most of the yolk 

 (the centrolecithal condition). Cytokinesis occurs between the periph- 

 eral nuclei, but the cells so formed remain open on the side toward the 

 yolk. Further divisions of these cells produce the ventral plate from 

 which most of the embryo arises. 



