908 ANATOMY AND DEVELOPMENT 



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 blasto- 

 pore of the embryo. The blastopore leads into the archenteron. 



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

 oval in form. Its length is 7 mm. The blastopore is elongated 

 and slightly narrowed in the middle. At the posterior 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 



Distance between front end of body and front end of 



blastopore - o6 



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. 



Stage D. The next stage is Balfour's stage, and has been 

 already described. 



The length is i - 34. 



It will be observed, on comparing it with the preceding em- 

 bryos, that while the anterior pair of somites in figs. 35 and 36 

 lie at a considerable distance from what we have called the 

 anterior end of the embryo (a), in the embryo now under con- 

 sideration they are placed at the anterior end of the body, one 

 on each side of the middle line. We cannot speak positively 

 as to how they come there, whether by a pushing forward of 



