187 



forms the whole of the endoderm. Each of these macromeres save the 

 left posterior one gives rise to a smaller endoderm cell one of which 

 comes to lie at the anterior end of the median furroAv while the other 

 two lie at both ends of the transverse furrow. These cells correspond 

 in origin and position to the primary mesoblast ; the latter however 

 unlike the smaller endoderm cells divides into a right and a left half 

 and each moiety proliferates cells forming a short mesoblastic band 

 Figs. 4 and 5). 



Four characteristically arranged ectoderm cells occupy the center 

 of the ectodermic area and when 36 ectoderm cells have been formed 

 these four central cells form the center of a cross of ectoderm cells. 

 One arm of the cross is anterior, one posterior, one right and one left. 

 In further development all the arms lengthen and all save the poste- 

 rior divide longitudinally into two parallel rows of cells (Fig. 5) . 



Fig. 4. 



Fig. 5. 



The cells of the posterior arm enlarge greatly and are carried for- 

 ward until they lie over or even anterior to the cross furrow while the 

 point at which the polar bodies are attached (the center of the cross) 

 is carried forward through an angle of about 90° so that it finally lies 

 at the anterior end of the long axis of the embryo. The position which 

 the polar bodies first occupied (immediately over the cross furrow) 

 coincides with the middle of the dorsal area while the ectoderm cells 

 which immediately surround the ectoderm pole are carried forward 

 until they lie at the cephalic pole of the embryo. The endoderm seems 

 to take no part in this shifting and the ectoderm on the posterior side 

 of the ovum is not shifted forward but grows around in the opposite 

 direction. There is thus a stationary point in the ectoderm on the 



