100 GENERAL CONSIDERATIONS. 



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 novr 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 

 caution in dealing with the general importance of the phe- 

 nomenon. 



I shall assume, then, to start with, that the ovum of the 

 Cape species has only recently lost its yolk, and that it may 

 be compared to an ovum of the New Zealand form from which 

 the yolk has been almost completely dissolved out by some 

 reagent. As a matter of fact, it is impossible, with our 

 present methods, to effect this complete solution of yolk and 

 leave its protoplasmic framework ; but what we cannot effect 

 has been done by nature in the most complete manner, leaving 

 an ovum which is little more than a loose protoplasmic spouge- 



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

 (Nos. 30, 31), and of a South American species wiiich 1 have had an opportunity 

 of examining, is relatively minute (diameter -04 mm.) and poor iu food-yolic. 



