THE ANATOMY AND DEVELOPMENT OF PERIPATUS NOVA E-BRIT ANNI AE. 35 



ment had assumed two divergent directions whose extreme points are represented in the 

 Neotropical and New Zealand species respectively. 



The starting point, according to Kennel, was to be sought for in an ancestral form 

 which discharged its small yolkless eggs directly into the water. The eggs would develop 

 into free-swimming larvae which fed themselves independently. Concomitantly with the 

 adaptation to a terrestrial life and the modification of organisation (e.g. development of 

 tracheae) which rendered oviposition in water impossible, the oviduct assumed the role 

 of a brood-chamber, as indeed in many other animals, e.g. Salamandra atra. 



At first, says Kennel, it may well be assumed that the intra-uterine development 

 was only slightly different from the free development ; the embryos and larvae would 

 be nourished by the uterine secretions as in Paludina vivipara, until finally all larval 

 structures required for a free life completely disappeared. This condition is represented 

 in P. capensis, although there is here, according to Kennel, no longer any identity with 

 the ancestral form. 



The rest of Kennel's conclusions on this subject are necessarily coloured by his inter- 

 pretation of the embryonic vesicle of the Neotropical species, as being a uterine and not 

 an embryonic structure (see above, p. 31). This does not however affect the principle of 

 his views. Referring to the two divergent methods of development mentioned above, he 

 says that in the one direction the nutrition of the embryo (at the maternal expense) 

 would be relegated to earlier and earlier stages and limited to a shorter time, until 

 finally a considerable quantity of nutritive yolk was collected in the egg itself, as in 

 P. novae- zealandiae. In the other direction the embryos became practically parasitic and 

 became applied 1 to the mucous membrane of the uterus. This is indeed true of the 

 Neotropical species and of P. novae-britanniae. In both cases the embryonic vesicle in life 

 is obviously closely pressed against the uterine wall, in the former without the inter- 

 vention of an egg-membrane, and in the latter with the egg-membrane separating the 

 ectoderm of the trophic organ (i.e. the trophoblast) from the uterine epithelium. As far 

 as our present methods enable us to judge, Peripatus must have had an aquatic 

 ancestor, and its viviparous habit must have been preceded by an oviparous habit. 

 Assuming the latter to be true, namely, that Peripatus had an oviparous ancestor, 

 it is quite certain, to my mind, that the oviparity of P. oviparus Dendy, is a 

 secondarily acquired habit and not in any way to be confused with the primitive 

 deposition of alecithal ova. 



The accumulation of yolk in the egg of P. novae-zealandiae would lead by a com- 

 paratively simple gradation to a secondarily acquired habit of oviposition on terra firma, 

 the egg being provided with sufficient yolk for the nutrition of the embryo and surrounded 

 by a protecting envelope or egg-shell. It is therefore a most interesting fact that this 

 step has been taken by the Victorian .species of Peripatus, recently described by Dendy 

 as a distinct species, P. oviparus (Dendy 3). 



In P. oviparus, according to Dendy's discovery, the yolky eggs are normally laid, 

 and Dendy has succeeded in hatching out at least one embryo from such a deposited egg. 



In P. novae-zealandiae the eggs are sometimes abnormally discharged, as observed by 

 Hutton, but such precocious eggs do not develop further, so far as is known. 



1 Kennel says they sucked on to the mucous membrane. 



5—2 



