EMBRYOLOGY OF CLEPSINE. 281 



and c? In stage 37 free nuclei were found which were regarded 

 as descendants of tlie original nuclei of the blastomeres. 



These superficial nuclei go on multiplying by division during 

 the whole period of the epiboly. finally they are seen as mere 

 white dots scattered over the entire surface of the yolk. Six to 

 seven days after exclusion the entoderm-celb make their appear- 

 ance as clear cells with small nuclei, in the periphery of the yolk 

 already cut up into compartments by the septa. What hypo- 

 thesis is more probable than that these cells originate from the 

 free nuclei? My sections have convinced me that these entoderm 

 cells arise in the surface of the yolk, and that they do not 

 originate in the products of the blastodisc. To account for 

 their origin on the hypothesis of generatio equivoca is quite as 

 unnecessary as unsatisfactory. The view I have otFered above 

 seems to be the only way to account for all I have seen. The 

 positive proof, however, is wanting, and may be difficult to 

 obtain ; but I hope, as soon as fresh material can be had, to 

 trace the history of the superficial nuclei farther. The 

 muscular walls of the alimentary canal are probably derived 

 from the same mesoderm- cells which build the septa. 



c. Final position of the " residual yolk." — From what has 

 been said it will be seen that the '' residual yolh ^' (Lankester) be- 

 comes inclosed by the permanent entoderm. Here it is gradually 

 dissolved and assimilated. It is the only food the young worm 

 has before it abandons its parent. This is proved by the fact 

 that the development completes itself in the same manner and in 

 the same time when the embryo is removed from the parent. 

 According to Biitchsli (^-^t) ^^icl Robin, the three large blasto- 

 meres in Nephelis lie outside of the entoderm, and play a very 

 subordinate roll, if any at all, as '' food material.^' It is 

 almost, if not quite, certain that these large cells correspond to 

 those designated as a, h, and c in Clepsine. It is quite 

 remarkable that the remains of these cells should be found in 

 oji£..ease_(Clepsine), in tile entoderm, and in the other (Nephelis) 

 in the body-cavity. Furthermore the early appearance of the 

 entoderm in Nephelis is in marked contrast with what happens 

 in Clepsine ; but the contrast is somewhat diminished on the 

 supposition that the entoplasts, which I have described in the 

 latter, represent the nuclei of the future entoderm cells. Such 

 a formation of the entoderm is easy to explain from a compara- 

 tive standpoint. The less deutoplasm an egg contains, the 

 longer the total and regular cleavage continues. With the 

 accumulation of the same the more sluggish becomes the 

 cleavage, until a point is reached where the dividing nucleus 

 has no longer sufficient power to sever the entire mass. Thus 

 we arrive at the discoidal and peripheral cleavage. In Clepsine 



