282 c. o. WHITMAN. 



we have a combination of jail kinds of cleavage. At first it is 

 total aiKr'ijuil'e~fegurar ; then it becomes unequal^ and then 

 discoidal, and, so far as the blastomeres, a, h and c, are con- 

 cerned, peripheral. The cleavage power of the nuclei in a, h 

 and c is no longer sufficient to overcome the resistance of the 

 masses of yolk, and hence they begin a process of free division, 

 precisely as in the eggs of insects, where this sort of division 

 prevails from the outset. That these three segments [a, b, and c) 

 should furnish the entoderm, is in harmony with what Biitschli 

 has observed in Nephelis, and also with other cases of unequal 

 cleavage, where the entoderm arises from the larger of the 

 cleavage products. 



The formation of the entoderm in Euaxes (-r5--Tir) ^s essen- 

 tially the same as in Clepsine, the only difference being that in 

 the former the yolk is broken up into a larger number of primary 

 spheres. The nuclei pass from the centre of these spheres to 

 the outer surfaces, precisely as in Clepsine, and here finally be- 

 come the centres of the entoderm- cells, leaving the residual yolk 

 in the aliment cavity. The same result is accomphshed in 

 Astacus fluviatilis {-rb\--TTT) ^^ ^ \\ii\Q different way. Here 

 the entoderm-cells are at first within the yolk, but ultimately 

 outside of the same. The passage from one condition to the 

 other is a curious process, which, according to Dr. Reichenbach, 

 is accomplished in the following manner : The interiorly placed 

 entoderm-cells devour the yolk by means of amoeba-like pseudo- 

 podia, which they throw out around the yolk-elements. At length 

 the entire yolk becomes included w/Mi?^ the entoderm-cells, which 

 now have a long pyramidal form, the bases of which lie in the 

 outer surface of the yolk, and the apices form the boundary of the 

 gastrula-cavity (Archenteron). During this ])rocess of lengthening 

 ^outwards at the expense of the yolk the nuclei shift tlieir~position, 

 passing from the apical to the basal ends of the pyramidal cells. 

 In this position the cell-protoplasm gathers around them, and 

 finally splits off from the deutoplasmic portions. Thus the yolk 

 is finally inclosed within the entoderm, as in Clepsine and 

 Euaxes= 



According to Eabl (0774-!^) a similar splitting of the entoderm 

 takes place in the fresh-water Pulmonates, in consequence of 

 which the vitellus nutritivus is enclosed in the ca'lom.^ 



The same position of the residual yolk occurs in many other 

 MoUusca (Nassa and Eusus, Bobretzky (-rrv/i-TTT))' ^^^^ Ptero- 



' According to Lankester (" Ou the Development of tlie Pond-Snail," 

 ' Quart. Journ. Mic. Sci.,' vol. xxii (n. s. xiv), p. 384 — 5) tiiis is incorrect. 

 Primitively (Ijymn;vu.s) tlio tnhole entoderm forms the wall of a bilobed 

 cavil Y — t lie arclienlcron. Later flic mctamorplioscd "gastrula-endoderm- 

 cells" lie ou each side of I he "stomach," where they arc "eventually 

 absorbed as nutritive matter by diverticula of ihc alimentary canal, which 

 give rise to the liver." 



