4 THE DEVELOPMENT OF 



paper), at first lie just beneath the germinal area but later 

 sink into the yolk, where they finally disappear without tak- 

 ing part in the formation of any organ of the adult. The 

 mesoderm is said to arise from cells budded inwards from 

 the germinal bands. 



Nusbaum's account is confessedly preliminary and in de- 

 fault of figures of the sections on which he bases his con- 

 clusions, I should have more hesitation in assuming that 

 he had confused the parts of his embryos to a great extent, 

 were it not that a similar fatality had characterized his work 

 in other forms (cf. Whitman, '86, Clepsine ; Grosglik, '87, 

 Oniscus, etc.) If, as I have suggested in another place, 1 

 we assume that Nusbaum has regarded the abdominal flex- 

 ure as the blastopore, a portion of his results are readily 

 harmonized with those of other students. As will be no- 

 ticed, from his abstract given above, he places the position 

 of the blastopore in front of the place where the abdomen 

 is subsequently to form. But, so far as I recall, not a sin- 

 gle other observer agrees with him in this respect. The 

 universal concurrence of opinion, among those who have 

 carefully studied the subject is, that in the Podophthalmia 

 at least, the blastopore is behind the tip of the abdomen. 

 The early stages of the formation of the abdominal flexure 

 certainly do simulate an invagination and have apparently 

 been interpreted by Ishikawa('85)as the first inpushing of 

 the proctodeum, to which reference will again be made. 

 In sections of eggs which have undergone contraction dur- 

 ing the hardening processes, the space between the folds of 

 the ventral abdominal ectoderm is obliterated, and Nusbaum 

 may have readily interpreted the tissues thus pushed in as 

 a solid entoderm. If the position here taken be valid, all 

 of Nusbaum's other conclusions as to the germinal layers 



1 American Naturalist, xxi, p. 294, March, 1887. 



