THE AMERICAN SPECIES. 



173 



have received another interpretation. Although this latter has been opposed 

 by v. Kennel for very important reasons (No. 5), it has been adopted by 

 Sclater, and seems to have a certain value in so far as it affords some 

 explanation of the peculiar early developmental stages. 



According to Sclater (No. 9), cleavage gives rise to a blastula formed of 

 large cells, and containing a small cavity (Fig. 82 A). An invagination then 

 takes place in this (pseudogastrula, Fig. 82 B). The invaginated part alone 

 yields the embryo (Sclater)) while the outer layer, by the peculiar growth 

 of its cells, sejiarates from the embryo and becomes very thin, thus forming a 

 membrane which envelops the embryo (Fig. 82 C, a). From the embryo itself 

 another envelope arises, by the splitting 

 off of single cells, this latter corresponding 

 to the amnion described by v. Kexnel. 



The figures given by Sclater agree on 

 the whole with those of v. Kennel, but 

 they are interpreted by the two authors 

 in an entirely different way. What v. 

 Kennel regards as uterine epithelium is 

 considered by Sclater as an embryonic 

 envelope, for this no doubt is the meaning 

 of his pseudogastrula. Fig. 79 B (v. 

 Kennel) must therefore be regarded as 

 the stage of invagination corresponding 

 to Fig. 82 B (Sclater' s pseudogastrula), 

 and Fig. SO must be interpreted in a 

 similar way. Fig. 79 A, according to this 

 view, should be regarded as an older stage, 

 similar to that represented in Fig. 82 C. 

 Further, the two stages in which the 

 conjectural vesicle has either thin or 

 thicker walls ought not to be unhesita- 

 tingly derived one from the other, as is 

 done by Sclater. Indeed, far stronger 

 proofs must be brought forward for the 

 view adopted by Sclater before it can be 

 finally accepted ; it nevertheless appears 

 to us worth mentioning because it seems 

 best to account for the origin of the 

 embryonic envelopes which are attributed 

 to Pcripatus.* In any case, the two en- 

 velopes which are said to surround the 



embryo suggest the double embryonic envelopes (amnion and serosa) of the 

 Insecta, all the more that this double embryonic integument may have arisen 

 here as there by the formation of folds in the blastoderm. The position of the 

 embryo in relation to the folds might even correspond to that of the Insectan 

 germ in relation to the embryonic integuments, but we know too little of the 



* [Willey's observations on the development of P. novac-britanniac (App. 

 to Lit. on Onychophora, No. II. ), in which he finds that the egg gives rise to a 

 large, thin-walled vesicle (trophoblast) with a thickened invaginated embryonic 

 area, tend to support Sclater's views regarding the relations of embryonic 

 envelopes in P. Edwardsii, and are opposed to those of v. Kennel.^To us 

 they appear conclusive on this point. — Ed.] 



Fig. SI.— Median section through a pear- 

 shaped embryo of P. Edwardsii (after 

 v. Kennel), ent, entoderm ; n, um- 

 bilical cord ; w, point of growth. 



