Segmentation of the Egg of Bdelhstoma 57 



It is very obvious from an inspection of the plates that the drawings of the blastO' 

 discs vary considerably in size. There is no record of the magnifications at which these 

 drawings were made. Part, at least, of this discrepancy can be accounted for by the varia' 

 tions in the size of the eggs as previously noted, and there is, of course, a possibility that 

 the drawings were not all made to the same scale. 



SUMMARY 

 In the early history of the egg of Bdellostoma stouti we find four things especially 

 worthy of note. 



1. The egg is very large, varying in length from 14.3 to 29 mm., the average of 70 

 eggs being 22.8 mm. with a width of about 8 mm. 



2. The egg is strongly telolecithal : i.e., most of the protoplasm is segregated into a 

 disc at the animal pole but it extends as a thin mantle over the entire surface of the egg 

 which is mainly composed of yolk. 



3. The disc of protoplasm at the animal pole is upraised to form the germinal hil- 

 lock, which at first is nipple-shaped, but through a gradual reduction in height and broad- 

 ening of the base it becomes dome-shaped or helmet-shaped. Its later history is obscure but 

 it probably flattens out to conform to the general contour of the egg since it is entirely 

 absent at the time of gastrulation. We do not know oi any other vertebrate egg having 

 such a germinal hillock. 



4. The mode of cleavage is discoidal but the marginal blastomeres are incompletely 

 separated from each other and from the general protoplasmic mass by radial furrows. 

 Early cleavage is limited to the germinal hillock but in later stages extends beyond its site. 



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