266 SPERMATOGENESIS AND OOGENESIS 



(4, En), and an outer (Ef), and the separation of the cells at 

 one pole to form a mouth (M) a gastrula is produced. 



But the same result might be arrived at in another way. 

 We have seen that an ordinary Mucor-hypha becomes under 

 certain conditions, multicellular (p. 160), that is, a single 

 multinucleate cell becomes divided into numerous cells after 

 its final form is attained. We have also seen that some of 

 the ciliate Infusoria, such as Opalina and Oxytricha, are 

 multinucleate (pp. 119, 118). 



Let us suppose an infusor-like animal (Fig. 67, A) with a 

 mouth (MtJi) and gullet, numerous nuclei () and the 

 usual large number of food-vacuoles (f. vac). Imagine the 

 food-vacuoles to unite into a single central cavity (B, f. vac') 

 opening externally by the gullet, and the nuclei (nu) to 

 arrange themselves in two layers around the cavity. It is 

 obvious that all that would be necessary to convert such a 

 form into a gastrula (c) would be for its protoplasm to divide 

 into as many cells as there were nuclei. 



It is thus quite conceivable that diploblastic animals may 

 have arisen either from colonies of unicellular zooids or from 

 solitary unicellular but multinucleate forms. At present 

 there is not sufficient evidence to allow of any decision being 

 arrived at on this point, but as the continuity of the animal 

 series cannot be grasped without imagining some such 

 intermediate stages as those described, it is thought advis- 

 able to put these two hypotheses before the reader simply as 

 hypotheses, either or both of which may any day be over- 

 thrown by the inexorable logic of facts. 



