THE DEVELOPMENT OF ASPIDOGASTER. 



177 



as if they had just divided) were to be distinguished. In 

 more advanced ova the shell was complete, but either color- 

 less or of a very pale-brown hue. In some of these the pri- 

 mary ova contained many nuclei and were imbedded in and 

 surrounded by a confused mass of accessory yelk-granules ; 

 while in others these granules were aggregated into a num- 

 ber of regular spheroidal masses (Fig. 43, JJ). 



As development proceeds, the accessory yelk-masses grad- 

 ually disappear ; the primitive ovum, now become the homo- 

 logue of the blastodermic disk or vesicle in other animals, to 

 all appearance increasing at their expense. At the same 

 time, clear rounded vacuoles in various numbers appear in its 

 substance ; but the nuclei of the germ, though very minute, 

 can, with proper care, be readily detected between these. In 

 the final stages the shell becomes browner, the vacuoles and 

 granules disappear, and the substance of the embryo appears 

 homogeneous. But, if carefully examined, the minute nuclei 

 become visible, especially if water be allowed to act on the 



FiQ.43.—Aspido(?a$ter conchicola.—A, section of the ovary: 1, its anterior end; 2, 

 germinal spot surrounded by a distinct wall ; 3. 4, a complete germinal vesicle 

 and spot ; C, a primary ovum ; 7>, voung state of a complete ovum ; the primary 

 ovum partially surrounded by yelk-granules and a shell ; B, complete ovum, with 

 the accessory yelk aggregated into spheroids ; E, vacuolated embryonic mass ; F, 

 embryo. 



tissue, and, if the shell be burst, and its contents poured out, 

 they readily break up into small but well-marked cells, each 

 with its nucleus. At the same time, the embryo takes on a 

 form not very distantly resembling that possessed by the 



