82 



UXEQ UA L SEGMENT A TJON. 



The original smaller cells continue in the meantime to divide and so form 

 a layer enclosing the larger, leaving exposed however the opening of the 

 pit lined by the latest products of the larger cells. 



FIG. 4'2. SEGMENTATION OF ANODON PISCINALIS. (Copied from Flemrning.) r. polar 

 cells?, v. vitelline sphere. 1. Commencing division into two segments ; one 

 mainly formed of protoplasm, the other of yolk. 2. Stage with four segments. 

 3. Formation of blastosphere, and segmentation cavity. 4. Definite segmen- 

 tation of the yolk sphere. 



The eggs of Anodon and Unio serve as excellent examples of the type 

 in which the ovum has a uniform structure hefore the commencement of 

 segmentation, but in which a separation into a protoplasmic and a nutritive 

 portion becomes obvious during segmentation. 



In Anodon 1 the egg is at first uniformly granular, but after impregnation 

 it throws out on one side a protuberance nearly free from granules 

 (fig. 42, i). 



In the case of this clear protuberance and of the similar protuberances 

 which follow it, the protoplasm is not at first quite free from food-yolk, 

 but only becomes so on being separated from the yolk-containing part of the 

 ovum. We must therefore suppose that the production of the clear 

 segments is in part at least due to the yolk spherules becoming used up to 

 form protoplasm. Such a formation of protoplasm from yolk spherules has 

 been clearly shewn to occur in other types by Bobretzky and Fol. 



The protuberance soon becomes separated off from the larger part of 

 the egg as a small segment composed of clear protoplasm. From the larger 

 segment filled with food-yolk, a second small clear segment is next budded 

 off, and simultaneously (fig. 42, 2} the original small segment divides into 

 two. Thus there are formed four segments, one large and three small j the 

 large segment as before being filled with food-yolk. The continua- 

 tion of a similar process of budding off and segmentation eventually results 



in the formation of a considerable number of small and of one large 



a 



Between this large and the small segments is 



segment (tig. 42, 3). 

 segmentation cavity. 



Eventually the large yolk segment, which has hitherto merely budded 

 off a series of small segments free from yolk, itself divides into two similar 

 parts. This process is then repeated (fig. 42, 4) and there is at last formed 

 a number of yolk segments filled with yolk spheres, which occupy the 

 place of the original large yolk segment. Between these yolk segments 

 and the small segments is placed the segmentation caA r ity. 



The segmentation of the ovum of Euaxes 2 resembles that of Unio in the 

 budding off of clear segments from those filled with yolk, but presents 

 many interesting individualities. 



1 Flemming, Enticick. Jcr Nojaden, Sit:, d. Akad. IF/.w. Wien, Bd. 4. 1875. 

 3 Kowalevsky, Mem. Akad. Peteraburp, Series vii. 1871. 



