GROWTH AND DIFFERENTIATION OF THE GERM-CELLS 11$ 



is one-half that characteristic of the somatic cells. 1 In both these 

 cases, the chromosomes are doubtless bivalent, representing two 

 chromosomes joined together. In Ascaris, in like manner, each of 

 the two chromosomes of the stem-cell or primordial germ-cells is 

 probably plurivalent, and represents a combination of several units 

 of a lower order which separate during the segmentation of the 

 thread when the somatic mitosis occurs. 



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 ovarian 

 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 ccelenterates, 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 

 Amoeba? More commonly, a definite association is established be- 

 tween the egg and the surrounding cells. In one of the most fre- 

 quent arrangements the ovarian cells form a regular layer or follicle 

 about the ovum (Figs. 58, 60), and there is very strong reason to 

 believe that the follicle-cells are immediately concerned with the con- 

 veyance of nutriment to the ovum. A number 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. 3 Such cases are, in any case, extremely rare ; and, 

 as a rule, the material elaborated by the nutritive cells is passed 

 into the egg in solution. Very curious and suggestive conditions 

 occur among the annelids and insects. In the annelids, the nutri- 

 tive cells often do not form a follicle, but in some forms each egg is 

 accompanied by a single nurse-cell, attached to its side, with which 

 it floats free in the body-cavity. In OpJiryotrocJia, where it has been 

 carefully described by Korschelt, the nurse-cell is at first much larger 



1 Cf. p. 194, Chapter V. 



2 It has been asserted that the eggs in such cases feed on the other cells by ingulfing 

 them bodily, Amoeba-fashion. This is probably an error. 



3 See Floderus, '95. 



i 



