20 THE ANATOMY AND DEVELOPMENT OF PERIPATUS NOVAE-BRITANNIAE. 



Sections through the trophoblastic or trophic vesicle of the present species in front 

 of the posterior ventral embryonic plate show that the wall of the vesicle consists of 

 two layers. The outer layer, the ectoderm, consists of cubical cells of moderate height 

 with vacuolar contents, each containing a large nucleus with usually two "nucleoli." 

 The nuclei of the trophoblastic ectoderm differ from those of the cells forming the 

 embryonic plate, in their staining properties; they take the stain (haematoxylin) less 

 deeply than do the latter. The inner layer, or endoderm, consists of a thin irregular 

 layer of protoplasm applied against the inner surface of the ectoderm and contains 

 scattered globular nuclei which project into the cavity of the vesicle. 



The cavity of the vesicle in the early stages is as a rule quite free from foreign 

 bodies of any description, but at a later stage (Stage VIII.) we shall find that it 

 contains many wandering endoderm cells with large nuclei containing a chromatin 

 reticulum with wide meshes — the latter character occurring frequently also in the 

 endodermic epithelial nuclei. These in-wandering cells may be called trophocytes, and 

 compared with the vitellophagous cells in the insect ovum. 



The ectoderm of the embryonic plate, except over the primitive streak, consists of 

 a high epithelium with large nuclei densely packed in several tiers. The endoderm here 

 does not differ materially from the peripheral endoderm ; sometimes the nuclei are set 

 more closely together than in the latter. 



Sections through the primitive streak of an embryo at this stage are given in 

 PI. IV, Figs. 39 — 40. Figure 39 passes through the primitive groove. As the series is 

 traced forwards this groove nearly flattens out until, as the anterior portion of the streak 

 is approached, another depression is met with (Fig. 40). This second depression may be 

 called the cranial groove, since in later stages it is bounded by the procephalic lobes. 



The interpretation which my preparations lead me to put upon them is that the 

 stomodoeal involution bears a similar relation to the cranial groove to that which the 

 proctodoeal involution bears to the primitive groove. 



The extraordinary resemblance of the embryo at this stage to an Insect embryo 

 with short superficial embryonic area on the ventral side of the egg, as occurs in some 

 Orthoptera, as also its likeness to any Insect embryo before the infolding of the embryo 

 (see Korschelt and Heider, 13, p. 774), cannot fail to strike the reader. 



When we come to compare the trophic folds of the embryo of P. novae-britanniae 

 with the amniotic folds of Insects, this remarkable similarity of the embryos should be 

 borne in mind (see below, p. 32). 



Stage IV. Figures 24 and 25 represent portions of two blastodermic vesicles in which 

 the embryonic area no longer has the posterior position seen in the preceding stage. 

 The trophoblastic wall of the vesicle has grown backwards beyond the embryonic plate, 

 so that the latter lies on the ventral side of the vesicle at some distance from the 

 posterior end. In Fig. 24 the embryonic tract has not yet arrived at its definitive location. 

 Fig. 25 is somewhat farther advanced, and it shows well two pits which denote the 

 positions of the primitive and cranial grooves respectively. The ventral surface which 

 commences in the next stage appears as a transverse groove occupying the region 

 between these two grooves. 



Figures 46 — 50, Plate IV, are taken from a series through the embryo represented 



