4 THE DEVELOPMENT OF 



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

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

 ing part in tbe formation of any organ of ihe adult. Tiie 

 mcsoderm is said to arise from cells biulded inwards from 

 the germinal bands. 



Nusbaum's acconnt 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 assuniing that 

 he had confused the parts of his embryos to agreat extent, 

 were it not that a similar fatality had characterized his work 

 in other forms(c/*. Whitman, '86, Clepsine ; Grosglik, '87, 

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

 we assume that Nusbaumhas 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 hehind 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, andNusbaum 

 may have readily interpreted the tissues thus pushed in as 

 a solid entoderm. Ifthe position here taken be valid,all 

 of Nusbaum's other conclusions as to the germinal layers 



^ American Naturalist, xxi, p. 294, Maich, 1887. 



