THE DEVELOPMENT OF A MOLLUSK 45 



The seventh cleavage (Model 12) divides the second quartet 

 of ectoblasts. 



The eighth cleavage (Model 13) consists of a second division 

 of the first quartet of ectoblasts. 



The ninth cleavage is unique, only one rather large cell, the 

 " mesentoblast," M.-E. being separated off from the left posterior 

 of the larger cells (D, Model 14). This new cell divides into two 

 (Model 15) and again into four parts (Model 16). The upper 

 two of these four cells, concealed on the model by the rim of the 

 dorsal plate, multiply rapidly by division, and the cells which are 

 formed from them make their way between the dorsal plate of 

 the ectoblasts and the large yolk-laden cells below. They will 

 form the future middle layer or mesoderm of the embryo, and 



Fig. d 



are known as the " mesoblasts."' After the separation of the 

 mesoblasts the remaining three large cells finally divide, giving 

 in all eight or nine large inferior cells, the "entoblasts," which in 

 time will form the inner layer of the embryo. 



At an early stage there are thus separated in the egg the 

 rudiments of the three layers distinguishable in the develop- 

 ment of all higher animal organisms : ectoderm, mesoderm 

 and entoderm. These may be diagrammatically represented as 

 in Fig. d. 



The ectoblasts by multiplication of cells soon extend over 

 the entire ovum until only a narrow pore is left on the lower 

 or ventral pole (Models 17, 18). Owing to the unequal rate of 

 this growth, the upper or animal pole is at the same time shifted 

 anteriorly till its angular distance from the lower vegetative 

 pole becomes on this side only 90° (Model 18). 



