270 LILIAN SHELDON, 



General Considerations. 



The investigations which I have made on the January 

 eggs of Peripatus novse-zealandiEe^ although the stages 

 examined were few, nevertheless throw a good deal of light ou 

 the subject of the early history of the development. In my 

 former paper (3) I remarked upon the strange dissimilarity 

 which existed from the segmentation stages up to quite late 

 ones between the three species of Peripatus whose develop- 

 mental history has been at all fully worked at. In the cases 

 of P. capensis and P. novae-zealandiae at all events this 

 remark now requires modification. The developmental history 

 of the latter is now fairly complete as far as the gastrula 

 stage, and up to that point its resemblance to that of P. 

 capensis is very marked. As I pointed out before (3) the 

 segmentation is very similar, the main differences being easily 

 accounted for by the presence of the yolk in the one species, 

 and its almost total absence in the other. I have now shown 

 that in the New Zealand species the ectoderm, which at first 

 covers only a portion of the ovum, gradually grows round 

 until only a small space on its ventral side remains uncovered, 

 and at this spot an invagination takes place forming the 

 blastopore, behind which in the middle line the primitive 

 streak and groove are present. In all these stages the resem- 

 blance to the corresponding ones of P. capensis is very 

 striking, the main difference consisting, as in the segmenta- 

 tion stages, in the presence of the yolk. This similarity is 

 clearly seen on a comparison of Mr. Sedgwick's figures on 

 Pis. IV and V (1) and my own (3). In fact it seems somewhat 

 strange that the almost total loss of the yolk, which must almost 

 certainly have been possessed originally by the Cape species, 

 should have apparently been accompanied by so few modifica- 

 tions in its development, since so important a change of con- 

 ditions might have been expected to exert a considerable 

 influence on the latter. 



Unfortunately there are many stages wanting between the 

 gastrula stage and the next one which I have described in my 



