56 CARL GOTTFRIED HARTMAN 



d. Comparison of early and later blastocysts and growth of the egg 



A comparison of figures H and J (text fig. 4) will show the 

 reader at a glance the changes which the blastocyst, the albumen 

 and the shell have undergone from the 50-celled stage to the 

 time that the entoderm formation is initiated. 



The entire egg has grown comparatively little in size — from a 

 diameter of 0.40 mm. on the average to about 0.50 by 0.60 mm. 

 in the largest specimen (text fig. 4 J). This increase is due 

 largely to the turgidity of the entire structure, the liquid within 

 the ovum pressing against the plastic wall, which gradually 

 responds. In the fresh state and in alcohol the blastocyst eggs 

 in both stages are practically spherical. The blastocysts shown 

 in figure 6 differ more in the preparations than they did in the 

 fresh state, on account of differences in amount of shrinkage. 

 Measurements of the thickness of shell membranes resulted as 

 follows: No. 40, 0.0035 mm.; No. 43, 0.0027 mm.; No. 144, 0.0023 

 mm. Practically no change in thickness of shell has taken 

 place since the 16-celled stage. 



The blastocysts themselves vary more in diameter than do the 

 shell membranes, chiefly because of the differences in quantity of 

 albumen still left unabsorbed. It will be noticed that absorption 

 of albumen proceeds most rapidly in the region of the blastodisc, 

 less at the opposite pole. This is as one would expect from the 

 very apparent difference in protoplasmic mass as well as from 

 differences in metabolic activity of the cells at the two poles. 

 The absorption of albumen continues in this proportion; so that 

 in bilaminar blastocysts, when the formative region comes to 

 lie against the shell, the non-formative region may still have a 

 greater or less thickness of albumen separating it from the shell 

 (text fig. 4 L) . The ratio of activity of the formative cells (and 

 the consequent thickness of the embryonic area) to quantity of 

 albumen absorbed holds true only in general terms. Thus B, 

 text figure 6, represents an egg with thickened embryonic disc 

 whereas considerable albumen is left in the egg; figure F, on the 

 contrary, shows little albumen as well as few cell elements in 

 the formative area. 



