224 



EMBRYOLOGY. 



When, in the Eabbit, the ovum, which has reached the uterus, 

 has here become metamorphosed into the blastula already described, 

 it is still enveloped by the zona pellucida. This in the meanwhile 

 has been distended into a. thin pellicle (prochorion), which is subse- 

 quently destroyed. 



The blastula, or blastodermic vesicle, expands rapidly, and fi-om 

 the fifth to the seventh day grows from 1-5 mm. to 5 mm. in diameter. 



In consequence 

 of this increase 

 in size the pro- 

 chorion on the 

 seventh and 

 eighth days is 

 so closely ap- 

 plied to the in- 

 ner surface of 

 the uterus that 

 it becomes more 

 and more diffi- 

 cult, and finally 

 impossible, to 

 detach the eggs 

 without injury. 

 For by the rup- 

 turing of the 

 prochorion, 

 which adheres 

 to the walls of 

 the uterus, the 

 delicate blas- 

 tula, which is 

 in close contact 

 with it, generally becomes injured and torn open, and thereupon 

 collapses, owing to the escape of its contents. The latter have also 

 sufiered changes which make the investigation more difficult, having 

 increased in consistency until they equal in density the albumen 

 of the Hen's egg. 



During the process of attaching itself, the embryonic fundament, 

 which at first is round, increases in size and takes on a more elon- 

 gated form. On the seventh day it becomes oval (fig. 130 ag), then 

 pear-shaped, and on the eighth, day acquires a more and more marked 



Fig. 130. — Embryonic fundament of the ovum of a Rabbit of seven days, 



from KoLLIKER. 

 0, Vascular area (area opaca) ; o,g, embryonic fundament ; pr, primitive 

 streak ; rf, dorsal furrow. 



