150 THE GERM-CELLS 



D. Growth and Differentiation of the Germ-cells 



I. The Ovum. 



(a) Grozvth and Niitfitioji. — Aside from the transformations of 

 the nucleus, which are considered elsewhere, the story of the ova- 

 rian history of the ^gg is largely a record of the changes involved in 

 nutrition and the storage of material. As the primordial germ-cells 

 enlarge to form the mother-cells of the eggs, they almost invariably 

 become intimately associated with neighbouring cells which not only 

 support and protect them, but also serve as a means for the elabora- 

 tion of food for the growing egg-cell. One of the simplest arrange- 

 ments is that occurring in coelenterates, where the ^gg lies loose 

 either in one of the general layers or in a mass of germinal tissue, 

 and may crawl actively about among the surrounding cells like an 

 Aniceba. In such cases (hydroids) the ^gg may actually feed upon 

 the surrounding cells, taking them bodily into its substance or fusing 

 with them^ and assimilating their substance with its own. In such 

 cases {Tnbnlaria, Hydra) the nuclei of the food-cells long persist in 

 the egg-cytoplasm, forming the so-called " pseudo-cells," but finally 

 degenerate and are absorbed by the egg. It would here seem as 

 if a struggle for existence took place among the young ovarian cells, 

 the victorious individuals persisting as the eggs ; and this view is 

 probably applicable also to the more usual case where the Q.gg is 

 only indirectly nourished by its brethren. 



In most cases, as ovarian development proceeds, a definite associa- 

 tion is established between the Q.gg and the surrounding cells. In 

 one of the most frequent arrangements the ovarian cells form a 

 regular layer ox follicle about the ovum (Figs. 59, 79), and there is 

 very strong reason to believe that 'the follicle-cells are immediately 

 concerned with the conveyance of nutriment to the ovum. A num- 

 ber of observers have maintained that the folHcle-cells may actually 

 migrate into the interior of the Q.gg, and this seems to be definitely 

 established in the case of the tunicates and mollusks (Fig. 75).^ 

 Such cases are, however, extremely rare ; and, as a rule, the material 

 elaborated by the nutritive cells is passed into the Qgg either in solu- 

 tion or in the form of granular or protoplasmic substance.^ An 

 interesting case of this kind occurs in the cycads, where, according 

 to Ikeno ('98), the egg-cell is connected with the surrounding cells 

 by broad protoplasmic bridges through which cytoplasmic material 

 flows directly into the egg-cell. 



Very curious and suggestive conditions occur among the annelids 

 and insects. In the annelids the nutritive cells often do not form 



1 Cf. Doflein, '97. "^ See Floderus, '95, and Obst, '99. * ^ P- 349- • 



