XI 



PHYLUM ARTHROPODA 



595 



this is followed by a long, narrow, coiled vas deferens. The two 

 vasa deferentia unite together to form a median tube the ductus 

 ejaculatorius opening on the exterior, in the same position as the 

 vagina of the female. The wall of the proximal part of the 

 ejaculatory duct is glandular, and secretes a substance forming 

 complicated cases which enclose masses of sperms to form 

 spermatophores . 



Development. The differences between the species of Peripatus 

 as regards the segmentation of the egg and the formation of the 

 germinal layers as described by various observers are very con- 

 siderable. Nearly all the species are viviparous, but in some the 

 egg, before the completion of embryonic development, is enclosed 

 in a well-formed shell, and in certain species the eggs pass out to 

 the exterior before the emergence of the embryo. In some species 





FIG. 491. Two early stages in the development of Peripatus novae -zealandiac. A, trans- 

 verse section of an ovum in which the yolk is nearly covered by the blastoderm (bl.) ; B, 

 transverse section of an ovum in which the blastopore (blp.) is formed. (After Sheldon.) 



the egg encloses a considerable amount of food-yolk, in others the 

 quantity of food-yolk is small, and nutriment is obtained from the 

 parent. 



In P. novcs-zealandicB there is a superficial segmentation. The 

 first segmentation-nucleus is itself superficial, and segmentation 

 results in the development of a number of nuclei, each with its 

 island of protoplasm, which arrange themselves on what is destined 

 to become the dorsal side (Fig. 491, A), opposite the site of the 

 future blastopore, while some pass inwards to the central part of 

 the ovum. The peripheral nuclei multiply rapidly and grow round 

 the yolk so as completely to enclose it except on a small space 

 (blastopore) in the middle of the ventral side (B). There a thicken- 

 ing takes place, and an involution of the lips of the blastopore 

 results in a sort of invagination, the floor of the invagination-cavity 

 being formed of yolk with scattered nuclei. 



