118 



GENERAL EMBRYOLOGY 



In the growth of the egg the chief aspects are those associated 

 with nutritive relations of the developing ovum to the adjacent 

 cells, especially in those forms whose eggs contain a considerable 

 amount of yolk. In the non-localized ovary such as that of the 

 Sponges and some Hydroids, the ovum grows at the expense 

 of whatever cells happen to be adjacent to it (Fig. 56). In the 

 common Hydra, as the ovum grows to be 

 considerably larger than these tissue cells, 

 it becomes amoeboid and actually ingests 

 these neighboring cells, digesting their sub- 

 stance and growing rapidly (Fig. 44) . The 

 nuclei of these ingested cells are relatively 

 indigestible and remain for some time scat- 

 tered through the egg cytoplasm. Among 

 all those forms with definitely localized 

 ovaries growth of the ova is accomplished 

 very differently. When the eggs are small 

 and contain relatively little food, no 

 special nutritive mechanism is developed, 

 the egg forming the food substances in its 

 own cytoplasm from materials drawn from 

 the circulating fluids in the cavities of 

 the ovary. Such eggs develop independ- 

 ently of the .neighboring cells intermingled 

 with the ova. 



FIG. 64. Longitudi- TST-U J.T r n i* i i 



nai section through VYnen the tully lormed eggs contain large 

 ^be^T amounts of food substance this is usually 

 marginaiis. obtained by one of two chief methods. In 

 K o7 Ch nttritive the simplest cases certain of the ovarian 

 ceils; o, ovum contain- 'cells adjoining the egg take on the charac- 



mg amoeboid nucleus .... ,. 



partly surrounded by tenstics ot nurse cells. These may either 

 substance contribute their own substance directly to 

 the ovum or they may become intensely 

 active, forming deutoplasm which is then drawn from them by 

 the growing ovum (Figs. 63, 64). There may be a single 

 nurse cell for each ovum (Fig. 65), or the nurse cells may 

 be scattered irregularly through the ovary so that several 



