516 ADAM SEDGWICK. 



a certain extent with the peculiarities in the segmentation 

 of Peripatus capensis both intrinsically and in relation to 

 other forms. I think, however, that the subject is of sufficient 

 importance to deserve a more detailed treatment than it was 

 possible to give it in that place. 



There can be but little doubt that the ovum of this species 

 possessed, at a period relatively not very remote from the present, 

 a large amount of good yolk ; that it resembled in fact in this 

 respect the ovum of the species now living in New Zealand, 

 and the ova of Arthropoda generally. The large size, 

 combined with the almost complete absence of food-yolk, 

 can, it appears to me, only be accounted for by sup- 

 posing that the ovum is passing from the large-yolked 

 to the non-yolked condition, and is intermediate between 

 the ovum of the New Zealand and that of the neotropical 

 species. The ovum^ of the latter species would on this view 

 have been derived from the large-yolked ovum of some remote 

 ancestor. 



There are other instances in the animal kingdom of small 

 ova which there is strong ground for regarding as having 

 been derived from large-yolked ova. The most conspicuous 

 example of this is perhaps that of the Mammalia. Within this 

 class we find both large-yolked and small ova ; and the investi- 

 gation of the former which is now being carried on by 

 Caldwell has particular interest inasmuch as it will show 

 more completely than has been possible hitherto how the 

 development is modified by the loss of yolk. Caldwell's 

 investigations are not yet published, and we do not therefore 

 know whether there is an ovum amongst the lower Mammalia 

 with the property — unique so far as I know — of the ovum of 

 Peripatus capensis, viz. the large size combined with 

 the almost complete absence of yolk. 



It is this peculiarity which, while it gives the cleavage of 

 the ovum of P. capensis a great interest, necessitates great 



^ The ovum of the Trinidad species which has been investigated by Kennel 

 (Nos. 13, 14), and of a South American species which 1 have had an opportunity 

 of examining, is relatively minute (diameter '04; mm.) and poor in food-yolk. 



