vii] YOLK, NURSE CELLS 109 



finds that these nuclei arise from minute chromatin granules 

 which increase in size and become vesicular, and that the 

 granules themselves are originally extruded from the egg- 

 nucleus at an earlier stage. 



A process which is similar in essentials to that found in 

 insects occurs in many Crustacea, in which some oocytes 

 grow at the expense of the rest and completely use them 

 up as they develop into mature eggs. One of the most 

 extreme cases of the kind is seen in the Annelid Opbryo- 

 trocha where the eggs develop in the coelom. In this animal 

 each young oocyte has attached to it a single nurse cell, 

 which is at first larger than the oocyte, and has an enormous 

 nucleus ; gradually, however, the egg absorbs the substance 

 of the nurse cell, until only a shrivelled vestige of it remains 

 attached to the full grown egg. 



Finally, it should be recalled that in the Platyhelminthes 

 the egg itself is without yolk, but the yolk is provided by 

 special glands which open into the oviduct or vagina. The 

 fully formed egg there becomes surrounded by a mass of 

 yolk-cells, and a shell is secreted by glands around the 

 whole group. The "egg" of Platyhelminthes is thus a com- 

 plex body consisting of a small ovum embedded in yolk- 

 cells and the whole enclosed in a shell. 



The outer coverings of various eggs differ greatly in 

 different animals, as might be expected in view of the widely 

 varying circumstances in which the eggs begin their develop- 

 ment. In a number of marine invertebrates the egg is 

 naked, or is covered only by a soft gelatinous envelope 

 which is easily penetrated by the spermatozoon. Such eggs 

 are generally shed into the water and fertilised by spermato- 

 zoa similarly discharged by the male. In Echinoids eggs of 

 this kind secrete a membrane immediately after being fer- 

 tilised. Many aquatic and most terrestrial animals have 



