254 ARTHROPODS. 



is it of mammalian gestation. The older embryos lose this "placenta," 

 but each lies constricted off from its older neighbour in front and younger 

 neighbour behind. When born the young resemble the parents except in 

 size and colour. In P. nova zealandice, the ova pass from the ovary into 

 the uterus in December, and the young are born in July— a long period 

 of gestation. 



{b) Male (of P. edwardsii). The male elements are produced in small 

 testes, pass thence into two seminal vesicles, and onwards by two vasa 

 deferentia into a long single ejaculatory duct, which opens in front of 

 the anus. In the ejaculatory duct the spermatozoa are made into a long 

 packet or spermatophore, which is attached to the female during coiJU- 

 lation. 



[While it is characteristic of Arthropods, in which the development 

 of chitin is so predominant, that ciliated epithelium is absent, it seems 

 that in Peripatus, which is much less chitinous than the others, ciliated 

 cells occur in some parts of the reproductive ducts, and perhaps also at 

 the internal funnels of the nephridia. This is indeed what one would 

 expect. ] 



Development of Peripatus. — There is a strange variety of development 

 in different species of this genus. Thus there is much yolk in the ovum 

 of P. noviz zealandicE, extremely little in that of P. capensis. In the 

 former species the yolk has a manifold origin ; it is said to arise 

 in the protoplasm of the ovum itself from the breaking up of the 

 germinal vesicle, from surrounding follicle cells, and from yolk present 

 within the ovary. In P. capensis and P. balfouri spermatozoa 

 reach the ovary, and there probably the ova are fertilised, but in P. nova: 

 sealandii2 the spermatozoa are confined to the receptaculum seminis near 

 which fertilisation seems to occur. In the maturation of the ova of P. 

 capensis and P. balfouri two polar bodies are extruded as usual, but 

 none have been observed in the case of P. noz'ie zealandia. 



In P. capensis the "segmentation" is remarkable, for true cleavage 

 of cells does not occur. The fully "segmented" ovum does not 

 exhibit the usual cell limits. It is a protoplasmic mass — or syncytium 

 — with many nuclei. Even when the body is formed the continuity of 

 cells persists, nor does the adult lack traces of it. To Professor 

 Sedgwick, this singular fact suggested the theory that the Metazoa 

 may have begun as multi-nucleated Infusorian-like animals. 



The gut appears as a large vacuole within the multi-nucleated mass, 

 and a gastrula stage is thus established. 



In the ova of P. novm zealandis, which have mxich yolk, a superficial 

 multiplication of nuclei forms a sort of blastoderm which spreads over 

 almost the entire ovum. The segmentation in this case has been called 

 centrolecithal (the type characteristic of Arthropods), but it is again 

 true that for a long time the cells do not exist as well-defined units. It 

 has been said, indeed, that "the embryo is formed by a process of 

 crystallising out in situ from a mass of yolk, among which is a proto- 

 plasmic reticulum containing nuclei." 



From these examples the student will perceive how difficult it is to 

 give a succinct account of the development of Peripatus. 



