274 DK. J. B. G.4TENBY ON THE GERM-CELLS AND 



prolono-ed osmication enables them properly to withstand contact with alcohol 

 and xylol in the subsequent embedding and staining. I have not hitherto 

 met with such delicate yolk, though that of the mollusc eggsis often very 

 easily destroyed. No other author has described the true yolk of sponge- 

 eggs. 



While the yolk granules are beautifully fine and regular in size and shape, 

 the mitochondria are often extremely coarse, and of irregular shape, and 

 may occasionally be abnormally large (PI. 20. fig. 13, M). 



In the unfertilized and unmatured oocyte, the yolk granules and mito- 

 chondria are evenly distributed in the endoplasm, as depicted in PI. 19. 

 fio-. 9, and, as will be seen, are subsequently evenly sorted out among ihe 

 blastomeres during cleavage of the ovum. 



15. Ectoplasmic and Endoplasmic Regions in the Egg. 



Even in the youngest oocytes one may notice that at an early stage a clear 

 ectoplasmic zone becomes differentiated from an inner or endoplasmic zone. 

 The ectoplasmic zone (PI. 19. figs. 5 and 9, PI. 20. figs. 10-13, PI. 21. 

 fios. 18-20) contains few or no vacuoles, is smooth, and is often drawn out in 

 the form of blunt pseudopodia or filamentous dendriform threads. The 

 inner or endoplasmic zone is vacuolated completely and has a fine frothy 

 appearance ; it is in this region that the cytoplasmic inclusions lie, granules 

 in the ectoplasm of the oocyte being rare or never found. Occasionally, in 

 preparations fixed in mixtures containing alcohol or acetic acid, the vacuoles 

 collapse, and the egg comes to have a curious radiation of fibres around the 

 nucleus (see Jorgensen's figures, 8 a). Eggs treated with silver nitiate 

 solution show the endoplasm browner than the ectoplasm, and in some cases 

 the difference in quality of ectoplasm and endoplasm is as marked as that 

 shown in PI. 19. fig. 5, END, ECT. In favourable cases not only young 

 oocytes, but amceboeytes, may be seen to possess ectoplasmic and endo- 

 plasmic zones (PI. 19. figs. 7 and 8). As in the case of the older oocyte the 

 cytoplasmic inclusions lie in the endoplasm, while the pseudopodia consist 

 mainly of ectoplasmic material. 



During development of the egg, all the blastomeres in most cases come to 

 have an equal portion of the ectoplasm. 



The ectoplasm can be traced through cleavage up to the formation of a 

 blastula (PI. 21. fie;. 21), but it soon either becomes absorbed or is invaded 

 by endoplasmic substance. In PI. 21. fig. 22, a young blastula is shown, 

 which still has distinctly marked' ectoplasmic regions. In a rare number of 

 cases, it was found that in young blastulse the cells of one side had less 

 ectoplasm than those elsewhere, but in no example could I show that this 

 inequality had any relationship to the formation of the flagellated and non- 

 flagellated parts of the amphiblastula. 



