434 



COMPARATIVE ANATOMY 



CHAP. 



development are met with, the youngest embryos close to the ovary, the oldest 

 near the vagina. The younger embryos are fastened in a peculiar manner to the 

 wall of the uterus, in the older embryos this placenta-like connection ceases, but 

 they lie in a closed sac formed by the epithelium of the 

 uterus. The position of the embryo in the uterus is 

 externally marked by constrictions of that organ. As 

 the embryos cannot travel along the uterus, the latter 

 itself grows at the part nearest to the receptaculum 

 seminis, while its last chamber shortens and degener- 

 ates ; in this way space is provided for the attachment 

 of new embryos to its wall. In copulation, which pro- 

 bably takes place only once, the semen enters the 

 receptaculum seminis. The eggs from the ovary which 

 have reached the receptaculum ovorum pass thence into 

 the uterus. 



In other species of Peripatus either the 

 receptaculum seminis or the receptaculum 

 ovorum may be wanting, and the eggs, which 

 from the first have been better provided with 

 nutritive yolk, do not attach themselves to 

 the wall of the uterus. 



2. Male genital apparatus (Fig. 296). 

 This is paired, with the exception of the ter- 

 minal portion, which opens outwardly at the 

 FIG. 297. Part of a uterus same place as the vagina in the female. Each 



<* the two tubular te ^ is continued into a 



cavity, and the embryo r (e) con- short vas effepens, which opens like a funnel 

 tained in it with its placenta (ep), into a vesieula seminalis. From this again 



arises a fine coiled vas deferens, which, united 



with its companion, enters a long coiled terminal portion, the tubular 

 duetus ejaeulatorius. In the proximal part of the latter an envelope 

 of complicated structure is secreted round the masses of spermatozoa, 

 and a spermatophore is formed. 



Ontogeny. The development of Peripatus Edwardsii is complicated by the 

 attachment of the embryos to the uterus wall, the latter undergoing considerable 

 changes and forming a closed brood chamber (Fig. 297) round each embryo. In 

 the case of each embryo an umbilical cord and placenta are formed, serving for its 

 nourishment. Attached by the cord the embryo projects freely into the brood 

 chamber. The side of the embryo turned away from the navel cord (which is a 

 process of the dorsal side of its future head) becomes the ventral side. Around the 

 embryo an envelope yielded by itself and called the amnion is formed, and is attached 

 to the inner surface of the uterus. As the embryo grows older, it gradually curls 

 up within the brood cavity. 



In Peripatus all those parts of the body which are metamerically or segment- 

 ally repeated (the mesoderm segments, extremities, nervous system, coxal glands, 

 etc.) develop and differentiate in the manner universally characteristic of the 

 segmented animals, i.e. progressively in order from before backwards. 



The Mesoderm is differentiated into two ventral symmetrical mesoderm streaks, 

 which unite posteriorly (at the edge of the blastopore) in a median zone, and in this 



