xn] ASCARIS, SAGITTA 175 



Somewhat similar chromatin-diminution has been found 

 by later investigators in other species of Ascaris (for ex- 

 ample, A. lumbricoides), but it has not been observed in a 

 number of other Nematodes which have been examined. 

 Although the whole of the chromatin is retained in the cell 

 which will give rise to the germ-cells, while some of it is 

 extruded from the nucleus in the other cells, experiments 

 (such as centrifuging the eggs during development) show 

 that some cytoplasmic factor determines whether diminu- 

 tion shall occur or not, for in eggs which have been centri- 

 fuged or are otherwise abnormal two cells may retain the 

 full complement of chromatin and both become primitive 

 germ-cells. The case may thus be compared in this respect 

 with the process that determines sex in Phylloxera, where 

 the presence or loss of a particular chromosome in the egg 

 determines the sex of the individual which develops from 

 it, but some pre-existing factor decides whether the chromo- 

 some shall be extruded or not. Phenomena in some ways 

 comparable with the chromatin-diminution of Ascaris are 

 found in the developing oocytes of certain Insects, but 

 before describing this it will be convenient to consider other 

 cases of bodies which appear to determine the origin of the 

 primitive germ-cells in the early embryo. 



One of the classical examples of the very early differenti- 

 ation of the germ-cells is the little marine organism Sagitta, 

 the systematic relationships of which are still quite obscure. 

 In the adult the coelom is divided towards the hind end of 

 the body by a transverse septum, and a longitudinal 

 septum separates each portion into right and left halves. 

 Just in front of the transverse septum on each side lie the 

 two ovaries, and the testes are in corresponding positions 

 behind it. Each ovary and testis is derived from a single 

 cell, and these four cells are formed by the division of two 



