io8 CYTOPLASMIC STRUCTURES [CH. 



gradually the oocyte increases in size while at the same time 

 the follicle cells become smaller, and seem not only to act 

 as channels for the supply of nutriment to the oocyte, but 

 also themselves to be used up in providing it. As was men- 

 tioned in the previous chapter, in some insects at least these 

 nutritive cells undergo all the earlier stages of the meiotic 

 phase, and only after reaching the pachytene stage become 

 differentiated from the-oocytes which they surround, while 

 at the same time the double chromosomes of their nuclei 

 break up into their univalent components. In insects there 

 are three distinct types of arrangement of the nutritive 

 cells (cf. Text-fig. 8, p. 74). The ovary consists of elongated 

 strands or egg-tubes, with oogonia or young oocytes atjthe 

 narrow apex, and the older eggs arranged in regular order 

 one behind the other to the lower end, where the tube is 

 continuous with the oviduct. In some insects, for example 

 Orthoptera, the eggs are enclosed in simple follicles, which 

 doubtless supply nourishment to them in their early stages, 

 and secrete the shell or chorion when they become full-sized. 

 In others, for example Coleoptera and Hymenoptera, each 

 egg is separated from the next by a group of nutritive cells, 

 so that the eggs and groups of nurse cells alternate along 

 the tube. Finally, in the Hemiptera there is a single large 

 mass of nutritive cells near the apex of the egg-tube, and 

 all the growing oocytes are connected with this by long 

 strands, along which the nourishment flows from the nurse 

 cells to the egg. 



A peculiar condition occurs in Hymenoptera, and possibly 

 some other insects, during the deposition of yolk in the 

 growing egg. Numerous small nuclei appear in the yolk, 

 especially around the edge of the egg, and presumably are 

 concerned with the process of yolk-deposition. GATENBY* 

 1 See also BUCHNER, 1918. 



