48 THE ANATOMY AND DEVELOPMENT OF PERIPATUS NOVAE -BRITANNIAE. 



Fig. 3G. Portion of uterus containing an embryo of about the same stage as the pre- 

 ceding, but varying considerably in appearance. For some reason or other the cephalic flexure 

 has not taken place, so that the antennae still point towards the vagina (i.e. away from the 

 ovary) as in earlier stages. The trophic vesicle is still more reduced than in the preceding 

 embryo. There is no prolongation of the vesicle overlapping the ventral surface, but merely 

 a prominent lobe in front of the head. The antenna is seen to arise from the prominent 

 procephalic lobe behind which is the oral papilla of the left side. The manducatory 

 appendages, being already enclosed, are not to be seen. Behind the base of the procephalic lobe 

 is to be seen a small knob-like structure projecting dorsally and slightly backwards into the 

 interior of the embryo. This is the stomodoeum. The abdominal flexure has not receded 

 back so far as in the preceding embryo. From specimen VII. x 12. 



Fig. 37. Enlarged view of the head of the embryo shown in Fig. 33 ; from the ventral 

 side. The vestibule leading into the stomodoeum is so deep that the latter cannot be seen 

 in this view. The antennae arise by a narrow base from the procephalic lobes ; at the sides 

 of the latter are to be seen the optic vesicles. Behind the procephalic lobes, the lips are 

 beginning to surround the manducatory appendages, and behind these are the oral papillae. 



Fig. 38. Dorsal view of same. The stomodoeum is seen to he immediately below the 

 dorsal integument ; its lumen is seen in optical section ; the lines radiating from it no doubt 

 represent the retractor muscles. From its position with relation to the procephalic lobes it 

 will be readily inferred that it projects dorsally and forwards. At a later stage (Fig. 36), 

 as we have seen, it projects dorsally and backwards, and at a still later stage, as in the adult, 

 it projects backwards. Thus is illustrated what I have described as the rotation of the 

 stomodoeum. At an earlier stage than the present the stomodoeum projects straight forwards 

 (see next Plate). 



PLATE IV. 



As the chief object of the figures on this plate is to assist in the elucidation of the 

 embryos represented on Plate III, it was not thought necessary to fill in the histological 

 details. The outlines were drawn with the camera lucida from sections 5 n thick cut trans- 

 versely to the long axis of the embryonic vesicle. The layers are indicated by different tints, 

 the dark tint denoting ectoderm, and the lighter tint the mesoderm and endoderm. Where 

 the endoderm is clearly distinct from the mesoderm, it is differentiated from the latter by a 

 still lighter tint. 



The trophic vesicle in the fresh embryo is distended with nutrient fluid contents, and, 

 after preservation, its walls appear more or less collapsed according to the relative rapidity 

 with which the diffusion between the contents of the cavity and the preserving fluid has 

 taken place. 



Figs. 39, 40. Stage III. 



Fig. 39. Posterior region of embryonic tract with primitive streak and primitive groove. 



Fig. 40. Anterior region of primitive streak (seven sections from preceding) with over- 

 lying cranial groove. The thickened ectoderm of the embryonic tract passes gradually into 

 the simple trophic ectoderm. In this embryo the wall of the trophic vesicle had collapsed, 

 so that its cavity appears much reduced. 



