22 THE ANATOMY AND DEVELOPMENT OF PERIPATUS NOYAE-BRITANNIAE. 



Figures 51 — 57 will sufficiently elucidate the structure of this embryo. Fig. 51 is 

 taken through the centre of the primitive streak which, as already described, now projects 

 in a tougue-like manner over the depressed ventral surface of the embryo. The greatest 

 number of somites which I have met with in a single transverse section was five on each 

 side in sections passing posterior to the primitive streak and tail-swellings, through the 

 region of the backwardly directed cornua of the crescent-shaped embryo. 



In consequence of the folding over of the primitive streak, the primitive groove now 

 appears to lie on the reversed side as compared with previous stages (cf. Figs. 39, 48 and 51). 



I would further direct special attention to the condition of the stomodoeum in this 

 stage. This is the first appearance of the true stomodoeum, and its lumen is enclosed 

 within the thickness of the ectoderm at the base of the cranial groove on each side of 

 which the praeoral lobes are commencing to project (Fig. 55). This enormously thickened 

 ectoderm is the rudiment of the cerebral ganglia, and only occurs through a few sections. 

 A section or two in front of that shown in Fig. 55, the ectoderm undergoes considerable 

 reduction in thickness, and the blind end of the stomodoeum is cut tangentially (Fig. 56). 

 Thus the stomodoeum precedes the proctodoeum in time of appearance, and this holds 

 good also for P. novae-zealandiae (Sheldon 20) and P. edivardsii (Kennel 11). 



Stage VII. (Plate III, Fig. 29). In this stage the primary symmetry of the 

 embryo is lost owing to the oblique direction into which the caudal end of the embryo 

 becomes bent as a necessary result of its continued growth. The first pair of somites 

 are now present as prominent lobes. They are not free however but attached by their 

 dorsal sides throughout their whole extent to the wall of the trophic organ. This 

 condition will be again met with in the next stage. 



Traces of the crescentic form of the embryo as seen in Stage VI. can still be 

 observed in this embryo. The primitive streak is the cause of the contorted shape of 

 the embryo. The free caudal extremity is now no longer directed forwards as it was 

 in the last stage but it is directed to one side and consequently the whole embryo is 

 twisted on to one side. The embryo is now in a state in which transverse sections 

 are of next to no avail. The total length of the embryonic vesicle shown in Fig. 29 

 was 4-25 mm. In Fig. 29 a another embryo belonging to this stage is shown in 

 which the primitive streak is directed quite transversely. The praeoral lobes and 

 the caudal process are the prominent features of the embryo at this stage. 



Stage VIII. (PI. Ill, Fig. 30). In this stage the continued flexure of the embryo 

 brought about by the growth of the primitive streak has resulted in the restoration of 

 a certain amount of symmetry in the topographical relations of the various regions. 

 Accordingly sections through an embryo at this stage are instructive. 



In addition to the more or less continuous endodermic layer which lines the wall 

 of the trophic cavity, the latter now contains numerous wandering amoeboid cells or 

 trophocytes, which have been mentioned above. These are endoderm cells which have 

 relinquished their epithelial connections and wandered into the cavity of the vesicle. 

 They are present in great numbers in this stage and up to Stage X. All the endoderm 

 cells appear to be potential trophocytes. 



In Fig. 30, the free-growing point or caudal extremity of the embryo is directed 

 backwards so that transverse sections will pass accurately through the primitive streak 



