358 



Mr. A. Sedgwick. 



[May 21, 



rudiments in this region remain connected with the surface ectoderm 

 much longer than do those of the hinder parts of the body. In fact 

 it may be said that the praeoral part of the central nervous system 

 never does separate from the ectoderm ivhich gives it origin, inasmuch as 

 the latter is invaginated in the form of two longitudinal furrows, which 

 soon become deeper, and eventually closed, in exactly the same 

 manner that the medullary groove of a vertebrate embryo is closed. 

 Two closed vesicles thus originate, one for each of the future cerebral 

 ganglia, These vesicles are lined by ectoderm, and by that particular 

 ectoderm from which the nerve elements of the ganglia originate : 

 they persist throughout life as the small hollow appendages of the sapra- 

 oesophageal ganglia, described and figured by Balfour in his memoir 

 (op. cit., Plate 17, figs. C, D). 



The eyes arise as invaginations of the sides of the nervous 

 thickenings of the praeoral lobes during the spiral stage. The invagi- 

 nations are at first shallow, but soon become deeper, and eventually 

 converted into closed vessels, the front wall of which (i.e., the wall 

 next the skin) forms the epithelium outside the so-called lens of the 

 adult eye, while the internal wall thickens, and remains continuous 

 with the cerebral ganglion, and gives rise to the retina. The enclosed 

 vesicle persists and apparently becomes filled by the structureless lens 

 of the adult eye, if the structure described as such be not a mere 

 coagulum produced by reagents. The eye of Peripatus is therefore a 

 cerebral eye. 



Stomodseum and Proctodeum. — The ectoderm grows inwards 

 for a short distance, at the front and hind ends of the alimentary 

 canal.. The anterior ingrowth gives rise to the lining of the pharynx, 

 and possibly — but of this I am not yet certain — the oesophagus of the 

 adult. The buccal cavity of the adult is formed, as I have already 

 pointed out, by a fold of the integument which encloses the jaw and 

 external openings of the salivary gland. The posterior ingrowth 

 forms the lining of the rectum. 



The Endodeem. 



The endoderm arises from the large segmentation cells which 

 arrange themselves round a cavity, the future mesenteron. 



The mesenteron, from its very first appearance, opens to the 

 exterior. This opening or blastopore gives rise, as described above, to the 

 permanent front and hind openings of the alimentary canal. 



The mesenteron possesses no glandular appendages of any kind 

 whatsoever. 



The Mesoderm. 



The mesoderm arises, as described above, from the undifferentiated 

 cells of the primitive streak of the early stages. It is completely 



