SIZE OF OVA 569 



generally the perfection of the animal at birth is a feature which 

 pre-eminently distinguishes our genus and in which it is excelled 

 by no other animal. 



Development. Peripatus is found in Africa, in Australasia, 

 in South America and the West Indies, in New Britain, and in 

 the Malay Peninsula and Sumatra. The species found in these 

 various localities are closely similar in their anatomical charac- 

 ters, the principal differences relating to the structure of the 

 female generative organs and to the number of the legs. They 

 differ, however, in the most striking manner in the structure of 

 the ovum and in the early development. In all the Australasian 

 and Malayan species the egg is large and heavily charged with 

 food-yolk, and is surrounded by a tough membrane. In the 

 Cape species the eggs are smaller, though still of considerable 

 size ; the yolk is much less developed, and the egg membrane is 

 thinner though dense. In the New Britain species the egg is 

 still smaller ('1 mm.), and there is a large trophic vesicle. In 

 the neotropical species the egg is minute, and almost entirely 

 devoid of yolk. The unsegmented uterine ovum of P. novae- 

 zealandiae measures 1'5 mm. in length by '8 mm. in breadth ; 

 that of Eoperipatus Weldoni about 1 mm. in its longest dia- 

 meter ; that of P. capensis is '56 mm. in length ; and that 

 of P. trinidadensis '04 mm. in diameter. In correspondence 

 with these differences in the ovum there are differences in the 

 early development, though the later stages are closely similar. 

 The development has been worked out in P. capensis, to 

 which species the following description refers. The ova are 

 apparently fertilized in the ovary, and they pass into the 

 oviducts in April and May. In May the brood of the preced- 

 ing year are born, and the new ova, which have meanwhile 

 undergone cleavage, pass into the uterus. There are ten to 

 twenty ova in each uterus. The segmentation is peculiar, and 

 leads to the formation of a solid gastrula, consisting of a 

 cortex of ectodermal nuclei surrounding a central endodermal 

 mass, which consists of a much-vacuolated tissue with some 

 irregularly-shaped nuclei. The endoderm mass is exposed at one 

 point the blastopore (gastrula mouth). The central vacuoles 

 of the endoderm now unite and form the enteron of the embryo, 

 and at the same time the embryo elongates into a markedly oval 

 form, and an opacity the primitive streak appears at the 



