PERIPATUS CAPENSIS. 



icy 



Peripatus capensis. 

 In consequence of the eggs of Peripatus capensis being poor in 

 yolk, their cleavage is apparently total. According to Sedgwick, 

 an animal pole (corresponding to the later dorsal side) can be dis- 

 tinguished from a vegetative pole. Two meridional furrows divide 

 the egg into four blastomeres of equal size, each of which contains a 

 portion of the animal and a portion of the vegetative protoplasm. 

 These cleavage-planes are said not to cut through the whole egg, 

 the blastomeres being united centrally. At a later stage, an equatorial 

 furrow separates the smaller ectoderm-cells (animal pole) from the 

 larger entodermal blastomeres. At the close of segmentation, the 

 cells are very loosely connected, the smaller ectoderm-cells are closely 

 applied together, while the larger entoderm-cells are amoeboid and 

 scattered irregularly within the egg-membrane. The stage which 

 follows may roughly be compared with the blastula. The entoderm- 

 cells draw together and lie directly beneath the smaller ectodermal 

 cells, which then grow 

 round the entodermal 

 elements, a solid (and 

 therefore epibolic) gastrula 

 being thus formed in the 

 course of further develop- 

 ment. The archenteric 

 cavity is said to arise in 

 the entoderm through the 

 formation of "vacuoles"! 

 It opens externally at the 

 point which has remained 

 unaffected by the cir- 

 cumcrescence, and thus 

 corresponds to the blasto- 

 pore. Behind this, an 

 increase in number of 

 the cells of the superficial 



layer takes place, which leads to a thickening of this layer and 

 then to a separation of the lower layers, that have thus arisen, as 

 the mesoderm. During the lengthening of the blastopore, which 

 soon occurs, and the simultaneous increase in length of the whole 

 embryo (Fig. 84 A), the mesoderm grows forward on both sides of 

 the blastopore and thus yields the mesoderm-bands. The rudiment 

 of the germ-band is thus produced (Sedgwick). 



E. 





n Up. 



Fro. 



i. 



Pig. 75.— Section through a 16-celled embryo of /. 

 Edivardsii lying in the uterus (after J. v. Kennel). 

 E, embryo; i.Uw, inner -wall of the uterus; Ve, 

 uterine epithelium. 



