212 EDWIN G. CONKLIN. 



Although these different ooplasmic substances are chiefly 

 localized in certain regions of the egg, which give rise to certain 

 portions of the embryo, this segregation is not quite complete. 

 Most of the clear protoplasm is found in the upper (ectodermal) 

 half of the egg but some of it is also present in the lower half. 

 Most of the yolk is found in the lower (endodermal) half of the 

 egg, but a little of it is found in the upper half. Almost all of the 

 yellow protoplasm is located in the mesodermal crescent, but a 

 very small amount of it is found -around the nuclei of all the cells. 

 Thus samples of all of these egg substances are contained in all 

 of the cells ; nevertheless the segregation is so nearly complete 

 that the clear, the gray, the light gray and the yellow areas are 

 marked out with the greatest distinctness (Photos 7, 8). 



In the 4-cell stage, as shown by Photos 9 and 10, the distribu- 

 tion of these substances remains as in the 2-cell stage, the yellow 

 crescent being confined to the vegetal hemisphere and the pos- 

 terior quadrants, the gray crescent to the vegetal hemisphere and 

 the anterior quadrants, while the deep gray, yolk-laden substance 

 lies between these crescents at the vegetal pole and the clear pro- 

 toplasm occupies most of the animal hemisphere of the egg. 



In the 8-cell stage the localization of these substances is the 

 same as in the preceding stages, the clear protoplasm lying above 

 the third cleavage plane and the other substances below it (Photo 

 1 1). The perfectly sharp boundaries of the crescent do not coin- 

 cide with any of the cell boundaries, gray substance being found in 

 the posterior dorsal cells above, below, and anterior to the crescent 

 (Photo 1 1). The clear notch in the posterior profile of the cres- 

 cent in Photo 1 1 is a cap of the same clear protoplasm which 

 gathered around the sperm head at its entrance and afterwards 

 lay at the middle of the crescent (Photo 3). In the 8-cell stage 

 this clear protoplasm takes the form of two caps on the surface 

 of the yellow crescent and adjoining, on each side, the median 

 plane. In this same stage a small amount of yellow protoplasm 

 may be seen around the nuclei of all the cells (Photo 1 1). This 

 perinuclear yellow substance is most abundant in the posterior- 

 ventral and in the anterior-dorsal cells ; in the former it lies chiefly 

 on the dorsal and lateral sides of the nuclei, in the latter on the 

 posterior and lateral sides. In subsequent divisions of these 



