DEVELOPMENT OF THE CAPE SPECIES OP PBEIPATUS. 523 



This is not a new view ; it is the old view of the origin of 

 the Metazoa, and has been held recently by Saville Kent 

 (No. 28j vol. ii, p. 480) and others. It is entirely in 

 accordance with the facts of the development of Peripatus 

 capensis. 



With regard to this development, we have to observe that 

 we cannot speak of cells till a comparatively late period 

 (Stage b), and that the intimate structural change underlying 

 the processes of growth of the young embryo is not an increase 

 of cells, but a multiplication of nuclei. First of all a cortical 

 layer of nuclei, lying in the peripheral layer and entirely sur- 

 rounding, excepting at one point, the vacuolated spherical mass 

 of protoplasm of which the embryo consists, is differentiated. 

 The central protoplasm, which contains a few nuclei and a 

 great number of large and small vacuoles to which the nuclei 

 have at first no special relation, protrudes from the point at 

 which the cortical nuclei are absent, as though to extend 

 itself in an amoeboid manner in search of food. This is the 

 solid gastrula stage (Part II, fig. 20). In it no cell outlines 

 are distinguishable, the whole embryo differing only from, 

 say Vorticella, in its large size, and the presence of a 

 definitely arranged layer of nuclei round its periphery. 



From this stage the coelo-gastrula is derived by the simple 

 process of the confluence of the larger central vacuoles to form 

 a single internal cavity, the establishment of the definite 

 opening of this cavity to the exterior, and the arrangement of 

 the central nuclei in the protoplasm lining it (Part II, figs. 23, 

 24 b). Later the mesoderm appears. It is derived from 

 some of the nuclei already present, which increase in number 

 and arrange themselves in the protoplasm around some of the 

 vacuoles which thus early become specialised into another 

 organ, the ccelom. 



Metschnikoff, who has done such important service to 

 biology in drawing attention to the physiological importance 

 of amoeboid cells in the organism, has been one of the most 

 prominent advocates of the view that the formation of the 

 gastrula by invagination is a secondary process. He considers 



