172 ONYCHOPHORA. 



circular thickening of the uterine epithelium, which, as the " uterine 

 placenta," enters into close connection with the former (Fig. 80, p.e 

 and p.u). The stalk connecting the embryo with the placenta 

 continues to narrow, and is described by v. Kennel as the "umbilical 

 cord." According to this account the embryo becomes closely con- 

 nected with the wall of the uterus, and a thickening of the connective 

 tissue layer of the latter takes place, causing a constriction of the 

 uterine lumen in front of and behind the region which contains 

 the embryo, thus forming a closed brood-cavity (Fig. 88, p. 179). 

 The amnion and the uterine epithelium are now separated from 

 the embryo by a considerable cavity (Fig. 80). 



The germ-layers begin to form by an active increase and a con- 

 sequent ingrowth of the cells which commences opposite the point 

 of attachment of the embryo (Fig. 80, w). In comparing the 

 development of P. Edicardsii with that of other species of Peripatus, 

 the point at which this ingrowth takes place will recall the accumula- 

 tion of cells in the blastoderm in P. novae-zealandiae, in which 

 invagination eventually occurs, and which at the same time represents 

 the first indications of the germ-band. In the South American 

 species this point of ingrowth, which in position corresponds to the 

 ventral side of the embryo (the latter is attached by its dorsal 

 surface), must be regarded as the blastopore. From this point the 

 ingrowth proceeds continuously, and fills the whole inner space of 

 the embryo down to the " umbilical cord " (Fig. 80). The cells 

 of the latter have shifted apart, leaving a central lumen, and have 

 become arranged into an epithelium such as is also found all round 

 the embryo, except at the point of ingrowth (Fig. 81). This outer 

 epithelium corresponds to the ectoderm. The further differentiation 

 of the germ-layers is said by v. Kennel to take place through the 

 appearance of a cavity in the more dorsal part of the central cell- 

 mass and the regular arrangement of the cells in its neighbourhood 

 (Fig. 81, ent). The cell-layer thus differentiated, the entoderm, is 

 in this way distinguished from the ventral cell-mass lying at the 

 blastopore, which represents the mesoderm. This latter remains 

 connected with the ectoderm for a long time, even during the later 

 changes of shape of the embryo, and at this point (w) new cell- 

 material is continually produced (v. Kennel, Sclater). 



In the above description of the first ontogenetic processes in P. Edicardsii, we 

 have followed the account given by v. Kennel liecause the material at his 

 disposal, with regard to quantity and state of preservation, seems to guarantee 

 the reliability of his statements, but it should be mentioned that these processes 



