210 EMBRYOLOGY 



A certain difference in individuals is manifested in the 

 manner of their reproduction. The latter consists in the 

 production of embryos in the axial cell. But these are of 

 different shapes ; vermiform and infusoriform (rhomboid) 

 embryos can be distinguished (Figs. 99 and 100). They 

 arise in different individuals, which, according to VAJf Bene- 

 DEN, are recognizable even by their outward form. The 

 nematogenous individuals are longer and more slender, the 

 rhomhogenous shorter and more compressed. 



According to Whitman, in addition to the forms that bring forth only 

 vermiform embryos, and which he designates as primary Nevmtogevs, 

 there also occur forms in which at first infusoriform and later vermiform 

 embryos are produced {secondary Nematogens). 



Development of the Vermiform Embryos.— There 

 can hardly be any doubt that the cells which constitute the 

 earliest fundament of the reproductive elements, and which 

 correspond to the genital cells of the other Metazoa, take 

 their origin by the division of the axial cell of the parent. 

 The products of this process of division are, however, not 

 equivalent ; moreover, the newly formed cells remain in the 

 axial cell (Fig. 99), whereby the appearance of an endoge- 

 nous cell-proliferation is produced. The production of the 

 germ cells begins very early, for even in embryos there is to 

 be seen inside the axial cell and behind its nucleus a new 

 cell undergoing differentiation, the first germ cell (Fig. 99 A), 

 and a second one soon arises in its anterior part (Fig. 99 B 

 and C). Their nuclei have very probably arisen by division 

 from the nucleus of the axial cell. Subsequently the latter 

 takes absolutely no part in the formation of new nuclei. It 

 appears to preside over the other cell functions only. The 

 two germ cells, on the contrary, begin to increase by division, 

 and soon furnish a large number of genital cells, from which 

 the embryos subsequently arise. 



The development of the germ cells, which are eventually 

 present in large numbers within the axial cell of the parent, 

 takes place in situ after the manner of cleavage. An epibolic 

 gastrula is formed here, as in the Orthonectidce, except that its 

 inner large cell remains undivided. It becomes the axial cell. 



