STRUCTURE AND DEVELOPMENTAL RATE 



141 



Lot B2 had now been for 70 hours at 7°C. These showed r.iany 

 irregular germ-discs, but some were fairly regular 16- and 32-cell stages. 

 Their condition was thus much the same as on the day before and 

 they had scarcely progressed at all during the 24 hours. These eggs 

 were now returned to room temperature. 



Fig. 4 A control embryo 48 hours old, the germ-ring only one-quarter over 

 the yolk, far behind the usual stage on account of the cool season. 



Fig. 5 A group of cleavage patterns 48 hours after fertilization and after 45 

 hours at a temperature of 5°C. Development is practically stopped. In many 

 of the two-cell stages large vacuoles, V, occupy the entire center of the cells. 



Fig. 6 An irregular partly undivided protoplasmic mass with blastomeres 

 at its ends, 48 hours old after 45 hours at 7°C. 



Lot B3 still had, after 70 hours at 9°C., high segmentation discs 

 about the 128-cell stage. The discs were normal in general appearance. 

 Thus at this temperature development continues, but at an extremely 

 slow rate. This lot was now also returned to room temperature. 



When the eggs were 96 hours, four days old, the control embryos 

 w^ere fully formed with prominent optic vesicles, the embryonic heart 

 was not yet visible, and there was no pulsation. These embryos were 

 thus scarcely up to the midsummer 72-hour stage, since the embryonic 

 heart beat is often fully established before such a time. The cool 



