377 



In addition to demonstrating the paired arrangement of the 

 embryos, this vesicle also makes clear a condition frequently met with 

 in not a few cases, viz., that one pair of embryos is sometimes much 

 more highly developed than the other pair. In this particular 

 vesicle, embryos 3 and 4 are fully twice as large as embryos 1 and 2. 

 This inequality in the development of the two pairs of embryos is 

 probably to be accounted for by the fact that the two primary buds 

 do not always start at the same time; and this suggests that the two 

 buds must develop somewhat independently of each other. 



10 



Fig. 10. Photograph of a transverse section taken across the upper third of 

 a vesicle which shows the four embryos well diiferentiated. Note that embryos 1 

 and 2 are not as highly developed as 3 and 4, and that they are still confined within 

 the common amniotic cavity or bay {B"). 



Towards the tip or free end of the vesicle from which Fig. 10 

 was made the amniotic tubes of the four embryos converge to join 

 in pairs the ectodermic vesicle, which remains small and insignificant 

 and has been called the common amniotic vesicle. This condition of 

 the vesicle is retained until rather late embryonic stages, in which 



