26 THE ANATOMY AND DEVELOPMENT OF PERIPATUS NOVAE-BRITANNIAE. 



Orientation of the embryo. The embryos without exception, from the youngest 

 in which the anterior and posterior poles are discernible, to the oldest, are placed in one 

 direction. That is to say, the polarity of the embryos is constant. The anterior end of 

 the embryo is invariably directed towards the vaginal end of the uterus. Thus, when an 

 embryo is lying in the ascending portion of the uterus, its anterior end will point towards 

 the head of the mother, and when it comes to lie in the descending portion of the uterus 

 it will head towards the posterior end of the mother. Peripatus offers an interesting 

 example of the comparatively late appearance of bilateral symmetry. There can be no 

 question of bilateral symmetry throughout the segmentation stages. In P. novae- 

 britanniae it probably appears coincidently with the formation of the trophic cavity. 



Transformation of the Trophic Cavity of the Embryo into the Gastral Cavity 



of the Adult. 



Although I have correctly stated above that the trophic cavity of the embryo 

 becomes the gastral cavity of the adult, the transformation of the one into the other 

 is not such a simple matter as might be supposed. The embryonic endoderm which 

 was largely used up in the production of the trophocytes in Stage VIII has to be 

 reconstituted, and this reconstitution is accompanied by some remarkable phenomena, 

 chief among which is the appearance of very numerous eosinophile globules in the 

 wall of the gut. I caunot attempt to give full details as to the processes involved 

 in the reconstitution of the wall of the gut, but can only indicate the broad outlines. 



The first indication of change in the endodermic lining of the trophic cavity that 

 I have observed, appears in Stage X. Here the endoderm with its scattered nuclei 

 is seen to separate from the ectoderm leaving a space between the two layers. The 

 space thus left between ectoderm and endoderm is the commencement of the definitive 

 body-cavity or haemocoel and in it are to be observed wandering mesoderm cells. 

 There is a fairly continuous somatic layer of mesoderm but no splanchnic layer at 

 all yet. The latter appears to be represented at first merely by the wandering 

 mesoderm cells. The somatic layer probably grew out from the somites between the 

 ectoderm and endoderm before the separation of the latter to form a space. I have 

 clear indications of this in my preparations. This observation coupled with that of 

 Sedgwick's nephridial end-sac (see p. 9) may I think be regarded as an indirect 

 corroboration of Sedgwick's account of the history of the somites in P. capensis. The 

 cells of the endoderm have secreted a fine basal membrane, the membrana propria of 

 the gut, to which they appear more or less loosely attached and from which they 

 project boldly into the gastral cavity. Indications are not wanting that the wandering 

 trophocytes apply themselves to this membrane and take part in the formation of the 

 gastral epithelium. 



In sections through an embryo belonging to my Stage XI, which is considerably 

 farther advanced than the preceding stage, the dimensions of the trophic or gastral 

 cavity are much more reduced, there is a wide body-cavity, and the endoderm cells 

 which in the preceding stage were described as projecting into the gastral cavity have 

 now attained a great height ('09 mm.) and moreover have secreted another membrane 

 — a cuticular membrane — at their free ends. There are no definite cell outlines but 



