I5O THE GERM-CELLS 



D. GROWTH AND DIFFERENTIATION OF THE GERM-CELLS 



I. The Ovum 



(a) Growth and Nutrition. Aside from the transformations of 

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

 rian history of the egg 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 egg 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 

 Amceba. In such cases (hydroids) the egg may actually feed upon 

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

 with them 1 and assimilating their substance with its own. In such 

 cases (Tubularia, 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 egg is 

 only indirectly nourished by its brethren. 



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

 tion is established between the egg and the surrounding cells. In 

 one of the most frequent arrangements the ovarian cells form a 

 regular layer or 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 follicle-cells may actually 

 migrate into the interior of the egg, and this seems to be definitely 

 established in the case of the tunicates and mollusks (Fig. 75 ). 2 

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

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

 tion or in the form of granular or protoplasmic substance. 3 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. 2 See Floderus, '95, and Obst, '99. 8 Cf. p. 349. 



