596 



ZOOLOGY 



SECT. 



In another species Peripatopsis capensis the segmentation is 

 total. A peripheral ectodermal layer becomes formed, enclosing a 

 central mass of cells the endoderm except at one point where a 

 small area, the blastopore, is uncovered. 



In accordance with the smaller size of the ova and the relationship 

 of the embryo with the wall of the uterus, the South American 



an. 



FIG. 492. Three somewhat later stages in the development of Peripatus eapensis, showing 

 the mode of closure of the blastopore and the appearance of the primitive segments. A, 

 stage in which the blastopore (bl.) has the form of an elongated slit ; B, stage in which the 

 blastopore is closing up in its middle part ; C, stage in which the blastopore has become 

 closed up except the anterior part which has gone to form the mouth (mo.), and the 

 posterior part which has formed the anus (an.) ; the whole embryo has now become strongly 

 curved towards the dorsal side. (After Balfour.) 



species show a totally different mode of development. The eggs, 

 which are almost entirely devoid of yolk, undergo a total and 

 tolerably equal process of segmentation. Even at this stage the 

 embryo, which increases considerably in size, appears to receive 

 nutrient lymph from the uterine wall. Later an intimate connection 



