216 ONTCHOPHORA. 



their late phylogenetic acquisition. The irregular distribution of 

 the tracheae in Peripatus, as contrasted with their regular arrange- 

 ment in the higher Tracheata, seems to indicate a lower condi- 

 tion of the tracheal system, and thus confirms the view that it is 

 newly acquired, and found, in Peripatus, to a certain extent in its 

 initial stage.* 



LITERATURE. 



1. Balfour, F. M. The anatomy and development of P. capensis. 



Quart. Journ. Micro. Sci. Vol. xxiii. 1883. 



2. Gaffron, E. Beitrage zur Anatomie und Histologie von 



Peripatus. Theil. i. u. ii. Schneider's Zool. Beitrage 

 Bd. i. Breslau, 1885. 



3. Hutton, F. W. On P. novae-zealandiae. Ann. Mag. Nat. Hist. 



(4). Vol. xviii. 1876. 



4. Kennel, J. von. Entwicklungsgeschichte von P. Edwardsii 



und P. torquatus. Theil. i. u. ii. Arb. zool. Inst. Univ. 

 Wiirzburg. Bd. vii. u. viii. 1885 u. 1886. 



* [A most important paper dealing with Peripatus lias recently been published 

 by Willey (App. to Lit. on Onychophora, Xo. II.). This author, in describing 

 the anatomy and development of a new Peripatus [P. novae-britanniae), advances 

 such a large series of new facts and fresh interpretations of old ones, that the 

 above account becomes very incomplete without a summary of his results, 

 consequently all students of the Onychophora should refer to this monograph. 



Unfortunately, the earliest stages obtained were not well enough preserved to 

 make out the process of cleavage. The egg is small (roinm. ), without yolk, 

 and enclosed in a remarkably thick egg-membrane. Willey's most important 

 discovery is that at an early stage the embryo has the form of an oval and, for 

 the. most part, thin-walled vesicle, only a portion of which gives rise to the 

 embryo, the latter forming a thickened postero- ventral area which becomes, as 

 it were, invaginated into the vesicle. The thin-walled vesicle, which is com- 

 posed of both ectoderm and entoderm, serves to absorb nourishment and to 

 protect the embryo. Willey regards it as physiologically analogous to the 

 blastodermic vesicle of the Mammalia, and adopts for it Htjbrecht's term, 

 trophoblast ; it is possibly homologous with the embryonic envelopes of the 

 Insecta. A comparison of "Willey's figures with those of v. Kennel and 

 Sclateu will show that this trophic vesicle is undoubtedly the homologue of the 

 thin-walled vesicle which encloses the embryo of P. Edwardsii, and which 

 v. Kennel derived from the uterine epithelium. Willey's observations thus 

 support those of Sclater on this point. The trophic cavity eventually becomes 

 the gastral cavity, but the embryonic entoderm which was largely used up in the 

 forming of wandering trophocytes ( = vitellophags) has to be reconstructed ; 

 during this process the entoderm undergoes histolysis, and yolk -granules appear 

 to be formed. Willey regards the yolk of P. novae-zealandiae as a secondarily 

 acquired structure, consequently he differs from our author's conclusions regard- 

 ing the primitive nature of that form, and considers it as one of the more 

 specialised species of Peripatus. He does not consider that the large empty 

 trophic vesicle of P. novae-britanniae indicates the former existence of yolk, 

 but refers to Hubrecht's conclusions regarding the similar condition in the 

 higher Mammalia which he thinks justify his view, that the presence of yolk in 

 the eggs of Peripatus is a very recent secondary condition. — Ed.] 



