86 CYTOLOGY CHAP. 



The reasonable explanation which Boveri offered is that the uncleaved 

 Ascaris egg, like the eggs of so many other animals, possesses an internal 

 cytoplasmic differentiation into an upper, or " animal," and a lower, 

 or " vegetative," pole. In the normal monospermic egg the first cleavage, 

 being horizontal, divides it into "animal" and "vegetative" blasto- 

 meres, the former being less rich in yolk than the latter. The 

 chromosomes in the " animal " blastomere (S) undergo diminution, those 

 in the " vegetative" blastomere (P) do not. The second cleavage divides 

 both blastomeres into two, and again the chromosomes in the cell nearest 

 to the " vegetative " pole alone remain intact. 



Dispermic eggs, as stated above, divide simultaneously into four cells, 

 and this may take place in one of three ways, according as to how the 

 spindles are arranged in relation to the polarity of the cell namely, 

 one P and three S cells, two P and two S cells, or three P and one S cell, 

 as illustrated in Fig. 40. Chromatin diminution takes place in all the S 

 cells but not in the P's, that is to say, it takes place in all cells which 

 fail to contain a portion of the cytoplasm from the vegetative pole of 

 the egg. 



A fair number of animals are now known in which the germ-track 

 is thus visibly marked out, the distinguishing marks being of the most 

 varied, and sometimes surprising, nature, though probably all ultimately 

 of cytoplasmic rather than nuclear origin. Thus in the crustacean 

 Polyphemus pediculus (Kuhn, 1911), when the egg is laid it has attached 

 to it the remains of one to three " nurse cells " (oocytes which have failed 

 to develop into eggs but have been absorbed by other oocytes). When 

 the vitelline membrane is secreted it is formed in such a way as to include 

 this little mass of nurse cell remains within the egg, and it soon becomes 

 embedded in the egg cytoplasm. As long as the included mass lies 

 passive, it follows that it can only be present in one of the blastomeres, 

 and consequently at the i6-cell stage it is found in one blastomere 

 and is absent from the other fifteen. At this stage the little mass breaks 

 up and becomes scattered through the blastomere in which it lies. Conse- 

 quently, at the next division of this blastomere, both its daughter cells 

 contain portions of it. These two cells are the primordial germ-cells. 



A useful summary of the various forms of germ-track differentiators 

 at present known is given by Hegner (1914 and 1915). There appears 

 to be nothing corresponding to the animal germ-track in the higher 

 plants (see, however, footnote to p. 144). 



E. PARTHENOGENESIS 



It is plain that the development of the egg without fertilization must 

 involve some modification of the general rule that the mature egg has 



