Microns 



Figure 319. — Photomicrograph of a live fertilized egg of 

 C. virginica after the formation of two polar bodies (top 

 of egg). High-phase oil immersion lens. 







Microns 



40 



Figure 320. — First cleavage division of the egg of 

 virginica 70 minutes after fertilization. Blastomere 

 (left) and CD (right). Polar body — ^'- 



C- 



ion. Blastomere AB 



on top. 



furrows which will intersect the egg irito four cells, 

 A, B, C, and D. At this stage the mitotic figures 

 are fairlj- large and the ckromosomes are in a 

 favorable position for examination. In the best 

 sectioned preparations of the cleaving egg, eight 

 daughter chromosomes were counted at the begin- 

 ning of anaphase (fig. 322). It would be, however, 

 premature to state that the diploid number of 

 chi'omosomes in C. virginica is 8 because on other 

 preparations 7, 9, and 10 were counted. 



The third division of each quadrant cuts the 

 cells in the equatorial plane and separates the 



Microns 



10 



Figure 321. — Beginning of second cleavage of the egg. 

 Viewed from the animal pole. Whole mount, Kahl^ 

 Feulgen stain. 



Microns 



20 



Figure 322. — Section of an egg of C. virginica at the second 

 cleavage. Beginning of anaphase. Kahle, Heidenhain 

 iron-hematoxylin. 



first quartet of micromeres, small ceUs at the 

 animal pole, from the macromeres, or larger cells 

 at the vegetal pole (fig. 323). 



At the fourth and fifth cleavages, resulting in 

 16- and 32-cell stages, the micromeres overgrow 

 the macromeres. Only one of the macromeres is 

 visible in the figure 324 and two in 325, which show 

 a side view of an oyster egg at these two stages of 

 development. 



Cell lineage, or tracing the developmental 

 history of the cleavage blastomeres through to 



EGG, SPERM, FERTILIZATION, AND CLEAVAGE 



345 



