262 PROFESSOR F. M. BALFOUR. 



embryo the nerve-cords still scarcely separated from the 

 epiblast.^ 



We have since found, in Balfour's material, embryos of a 

 slightly different age to that just described. Of these, three 

 (figs. 34, 35, 36) are younger, while one (fig. 38) is older than 

 Balfour's embryo. 



Stage A. — The youngest (fig. 34) is of a slightly oval form, 

 and its greatest length is '48 mm. It possesses a blastopore, 

 which is elongated in the direction of the long axis of the em- 

 bryo, and is slightly narrower in its middle than at either end. 

 From one end of the blastopore there is continued an opaque 

 band. This we consider to be the posterior end of the 

 blastopore of the embryo. The blastopore leads into the 

 archenteron. 



Stage B. — In the next stage (fig. 35) the embryo is elon- 

 gate-oval in form. Its length is "7 mm. The blastopore is 

 elongated and slightly narrowed in the middle. At the pos- 

 terior end of the embryo there is a mass of opaque tissue. On 

 each side of the blastopore are three mesoblastic somites. The 

 length of the blastopore is '45 mm. 



Stage C. — In the next stage (fig. 36) the features are much 

 the same as in the preceding. The length of the whole 

 embryo is "9 mm. 



The following were the measurements of an embryo of this 

 stage with five somites, but slightly younger than that from 

 which fig. 36 was drawn. 



Length of embryo ...... '74 mm. 



„ blastopore . . . . . "46 „ 



Distance between hind end of blastopore and hind end of body '22 „ 



„ „ front end of body and front end of blastopore '06 „ 



The somites have increased to five, and there are indications 

 of a sixth being budded off from the posterior mass of opaque 

 tissue. The median parts of the lips of the blastopore have 

 come together preparatory to the complete fusion by which the 

 blastopore becomes divided into two parts. 



• ' Comparative Embryology,' vol. i. p. 318. 



