266 Mr. L. A. Borradaile— The Outcome 



" The anterior region of the embryo is practically a punctum 

 fixum, and the contortion of the embryo in a later stage is 

 almost entirely due to the growth which is taking place at 

 the primitive streak " — the latter being at the hind end. 



The endoderm has a chequered history. In one of the 

 earlier stages " many endoderm-cells forsake their epithelial 

 position and become converted into wandering trophocytes." 

 Subsequently the endoderm reconstitutes itself and forms " a 

 fairly compact epithelial layer containing numerous eosino- 

 phile granules of varying sizes." Later on still this endoderm 

 again breaks up. " In young individuals the brightly 

 staining globules have entirely disappeared. The endoderm 

 does not form an epithelial layer, but consists of cells lying 

 loosely and freely in the gastral cavity, like the trophocytes 

 in the embryo." A reconstitution of the endoderm after 

 this second histolysis has not been observed. It is suggested 

 that histolysis of the endoderm is a periodically recurring 

 phenomenon in Peripatus. 



As to the general bearings of this history, the resemblance 

 of the embryo in the earlier stages to that of an insect before 

 the infolding, and of the trophic vesicle, " when the embryo is 

 flexed and the trophic organ covers its ventral surface as with 

 a cap," to the amnion of an insect is duly pointed out in the 

 present paper, and has since been the subject of an article in 

 the * Quarterly Journal of Microscopical Science.' But there 

 is another resemblance, even more interesting if less obvious 

 than that just referred to, on which Dr. Willey is at present 

 silent. 



The discovery of a new method of development in Peripatus 

 naturally suggests speculation as to whether the embryo is in 

 any way comparable with the trochosphere larva of Annelids. 

 Now the embryo in question is a vesicular creature, with a 

 greatly swollen preoral region, a ventral mouth-site, and two 

 ventro- lateral bands of mesoderm (hindward these two bands 

 become one), starting at the hind end in the neighbourhood 

 of the future anus, and thence proliferating. The adult form 

 is reached by the elongation of the hinder part of the body 

 concurrently with the formation of new segments at the hind 

 end and the reduction of the antero-dorsal vesicular region. 

 In all these points our embryo resembles a trochosphere. 

 The absence of the ciliated rings would, of course, be expected 

 in view of the loss of the free life. No serious difficulty is 

 presented by the absence of a blastoccel, this condition being 

 already known in various Polychpetes {Psygmobranchus &c.) 

 and in the Earthworms. The embryos of the latter group, 

 under the influence of altered conditions of nutrition, show a 



