100 UNEQUAL SEGMENTATION. 



third generation next divide, forming in all twelve small cells and four large. 

 The small cells of the fourth generation then divide, and subsequently the 

 four large cells give rise to four new small ones, so that there are twenty 

 small cells and four large. The small cells form a cap embracing the upper 

 pole of the large segments. It may be noted that from the third stage 

 onwards the cells increase in arithmetical progression a characteristic 

 feature of the typical gasteropod segmentation. 



In the later stages of segmentation the large cells cease to give rise 

 to smaller ones in the same manner as before. One of them divides 

 first into two unequal parts, of which the smaller becomes pushed in to- 

 wards the centre of the egg. The larger cell then divides again into two, 

 and the three cells so formed occupy the centre of a shallow depression. 

 The remaining larger cells divide in the same way, and give rise to smaller 

 cells which line a pit which becomes formed on one side of the ovum. 

 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. 42. SEGMENTATION OF ANODON PISCINALIS. (Copied from Flemming.) 

 r. polar cells. z>. vitelline sphere, i . 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 segmentation 

 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 before 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, 1). 



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. 



1 Flemming, "Entwick. der Najaden," Si'tz. d. Akad. Wiss. Witti, Bel. 4, 1875. 



