50 RICHARD EVANS. 



various genera of the Peripatidse differ from one another 

 to a remarkable degree as regards the early stages in 

 their development, and it is quite possible that the method 

 by which the yolk becomes surrounded by the blastula 

 cells in the young embryo of Eoperipatus conforms more 

 closely to that existing in most Arthropoda than it does 

 to that occurring in the other genera of the Onychophora. 

 On this theory, Eoperipatus, which, from the point of view 

 of external characters and internal anatomy, seems to be 

 more primitive than the other genera belonging to the family 

 Peripatidse, would have to be considered more primitive, as 

 regards the mode of circumcrescence of the yolk, unless it 

 be admitted that the method in question has originated twice 

 within the limits of the Arthropodan phylum, a view which 

 is in no way probable. 



The mesoderm seems to be formed exclusively from the 

 primitive streak, but the endoderm develops from the lips 

 of the blastopore by invagination. 



The Second Embryo. — The next embryo which will be 

 considered corresponds to the one represented in the second 

 figure on PI. 5, and is considerably more advanced than 

 that described in the foregoing pages, for it possesses the 

 rudiments at least of eight somites. 



The ectodermal layer is complete on the dorsal surface, 

 though thin, and possesses nuclei which appear either circular 

 or oval in transverse section. When they are oval in shape 

 their long axis lies parallel to the surface. The ectoderm, 

 which covers the ventral surface, and is situated between the 

 two portions of the divided blastopore, has similar characters 

 (PI. 7, fig. 10 e). Elsewhere the ectodermal layer is con- 

 siderabl}^ thickened, and its nuclei are arranged three or four 

 deep. The thickening of the ectoderm is most marked on 

 the cephalic lobes, where the nuclei are arranged in three or 

 four layers, and on the sides, where they are arranged in two 

 layers. The cephalic lobes and the lateral bands, both pro- 

 duced by the thickening of the ectoderm, are continuous 

 with each other, and probably represent the undifferontiate(i 



