602 ARTHUR. WILLEY. 



approach to clearness. In the following considerations, facts 

 and hypotheses are given indifferently. 



1. The trophoblast of the embryos of Peripatus novse- 

 britannise is a mucous membrane in virtue of its close 

 proximity to the uterine mucosa ; the chorionic membrane, 

 however, always intervening between the trophoblast and the 

 mucosa. This is a special case of Driesch's principle of the 

 dependence of function on position — '^ Die Zelle ist Funktion 

 der Lage." 



2. The eggs of P. novse-britanniae have retained the 

 primitive alecithality which must have characterised the eggs 

 of the aquatic ancestor of Peripatus, which presumably spawned 

 in the water. In connection with the neotropical species of 

 Peripatus, Kennel was also of the opinion that their lack of 

 yolk was a primary deficiency, and not a secondary loss. 



If this postulate as to the primary character of the aleci- 

 thality of the eggs of my Peripatus be not conceded, my theory 

 would be thereby practically rendered nugatory. 



3. The viviparity of Peripatus is to be regarded as a direct 

 result of the transition from an aquatic to a terrestrial life. 

 This was also Kennel's opinion. 



4. In adaptation to the requirements of the intra-uterine 

 nutrition of the embryo, various provisional embryonic trophic 

 organs were evolved, e.g. dorsal (ectodermal) hump of P. 

 capensis (Sedgwick), trophic vesicle of P. novse-britannisB, 

 trophic sac of P. edwardsii, yolk of P. novse-zealandise. 

 It is neither easy nor necessary to decide which of these came 

 first, or whether they were evolved more or less independently. 

 Future investigation of the development of other species, such 

 as P. tholloni, Bouvier, from the Gaboon'district, may throw 

 light on this matter.^ 



• For facts indicating the relatively primitive character of P. novse- 

 britanniae the reader may be referred to my paper (14). According to the 

 descriptions of Kennel and Sclater, the trophic organ of the embryos of the neo- 

 tropical Peripatus lias the form of a spheroidal vesicle, into which the embryo 

 is inverted and to the wall of which it is united by an at first solid stalk. The 

 vitelline membrane undergoes early resorption (Kennel), so that the wall of 

 the sac comes into direct contiguity with the uterine mucosa. This inversion 



