440 



INVERTEBRATA 



CHAP. 



The egg divides into two blastomeres, an upper and a lower. 

 The upper, which is designated AB, is slightly larger and freer from 

 yolk granules than the lower, which is designated P r The nucleus 

 of AB undergoes an extraordinary change known as the " diminution 

 of the chromatin " (Fig. 342, B). At the moment that it separates 

 from its sister nucleus in P p it contains, like the fertilized egg, four 

 chromosomes (or two in the variety monovalens"). But as it passes 

 into the resting stage, the greater portion of each of these chromosomes 

 is cast out as amorphous masses of chromatin, which for a time can 

 be recognized in the cytoplasm but which are gradually absorbed. 



The remainder of each chromo- 

 some breaks up into a number of 

 minute granules. 



When AB next divides each 

 of these granules acts like a 

 minute chromosome, and hence 

 the spindle that is formed ex- 

 hibits a totally different appear- 

 ance from the spindle that is 

 formed in P a , which shows, of 

 course, the ordinary four (or two) 

 chromosomes. 



At the next cleavage AB 

 divides into an anterior cell A 

 and a posterior cell B, whilst P l 

 divides into an upper cell de- 

 nominated EMST and a lower 

 cell P 2 . The nucleus in EMST 

 undergoes a reduction precisely 

 similar to that undergone by the 

 B, two nucleus in AB. The embryo is 

 now shaped like a T. The two 

 lower cells bend upwards first in 

 a plane at right angles to the plane of the two upper cells ; they 

 then, however, swing round into the same plane, and so the T becomes 

 a rhomb, and the plane of this rhomb is the future median plane 

 of the embryo. 



At the next cleavage A divides into a right cell named a and a 

 left cell named a. B divides similarly into a right cell named b and 

 a left cell named /3. EMST divides into an anterior cell named 

 MST and a posterior one named E. P 2 divides into an anterior cell 

 called P 3 and a posterior one called C. The nucleus in C undergoes 

 reduction so that only P 3 retains large chromosomes. The four cells 

 MST, E, P 3 , and C follow each other in a curve, convex below, which 

 lies in the median plane of the embryo. They and their descendants 

 are known as the ventral cell family, whilst a, b, a, and /?, and 

 their descendants, are known as the dorsal cell family. 



The dorsal cell family divides much more rapidly than the 



FIG. 342. The 2-cell stage of the egg of 

 Ascaris megolocephala (monovalens) when 

 the spindles for the 4-cell stage are being 

 formed, showing diminution of the chroma- 

 tin. (After Boveri.) 



A, the 2-cell stage seen from the side, 

 chromosomes from the upper cell enlarged to show 

 the process of diminution of the chromatin. 



