OBSERVATIONS ON THE MATURATION PROCESSES. 41 



perchen " now occupy the middle of the interzonal filaments. The spindle 

 is oblique to the surface of the egg, and one pole is so near the surface 

 (fig. 29a) that the peripheral mass of chromatin lies close to the edge of 

 the protrusion which is destined to be cut off to form the polar cell. The 

 constriction has begun on the side nearest the "Zwischenkorperchen," 

 the vitelline membrane being already in contact with the " Zwischenkor- 

 perchen" nearest the surface of the egg (fig. 296). The rest of the 

 process, involving the final separation of the polar cell, is as described 

 on page 34 for the first polar cell. 



C. RIPE EGG. 



Stage XI. — The Pronuclei. 



A discussion of the further development of the ripe egg does not 

 lie within the scope of the present work. It suffices to say that the chro- 

 matin mass resulting from the union of the chromosomes remaining 

 after the formation of the second polar cell is quickly transformed into 

 the egg nucleus. This usually occurs simultaneously with the develop- 

 ment of the sperm nucleus. But in two cases the egg nucleus had reached 

 a diameter of 6 micra, while the head of the spermatozoon had not been 

 appreciably changed in form or size. In no case has the sperm nucleus 

 been observed before the chromatin mass has begun to be transformed 



into the egg nucleus. 



D. POLAR CELLS. 



The observations on the polar cells here recorded do not extend to 

 the cleavage stages of the egg. Therefore, no statement can be made 

 concerning the further fate of the polar cells, or concerning the changes 

 which take place in the second polar cell. 



First polar Cell. 



The first polar cell, originating as described on page 34, is usually 

 an ellipsoidal or a flattened spheroidal body, the three diameters of which 

 are nearly always unequal. The average dimensions of 28 polar cells — 

 each of which had been recently formed (Stage VI), the first spindle 

 being still in the telophase (plate 4, fig. 18) — were 22.7 X 19.2 X 13.5 

 micra. These figures indicate the average size at its largest stage. With 

 age some polar cells diminish very rapidly in size (figs. 18, 32-37, plate 6) ; 

 others retain nearly their original dimensions. Disregarding for the pres- 

 ent the very small forms (figs. 35-37), it is found that the first 50 polar 

 cells (which could be measured most accurately) from 100 of the young- 

 est eggs which have the complete second spindle give as an average the 

 following dimensions in micra: 20 X 15.6 X 11.8; and 22 polar cells 

 (all that could be measured) from 100 of the oldest eggs of the same 

 stage give the following average dimensions: 16 X 13 X 10.5 micra. 

 These averages show a considerable decrease in size; and, as a series of 

 gradually diminishing sizes can be found down to that shown in fig. 37, 

 and as the smaller sizes are too numerous (55 out of 507 eggs) to be mere 



