PLATE I 



EXPLANATION OF FIGURES 



Note, figs. 26-29 represent a series of transverse sections of a five-somite embryo. 



26. A section through the region of the head-fold. The brain vesicle is in the 

 process of formation, and the neural groove (n.g.) has become very deep. The no- 

 tochord {n.ch.) is represented by a row of cells, and to each side of it the entoderm 

 is bayed to form the pharyngeal pouches (ph.g.). X 68. 



27. A section through the somite region. The somite shows a distinct cavity, 

 and the coelomic cavity is forming. The entoderm is beginning to close in beneath 

 the notochord. X 68. 



28. A section through the proximal part of the allantoic tube. The bands of 

 the belly-stalk have become much folded, and contain a number of umbilical blood 

 vessels. The posterior amniotic process has become reduced to a very small tube. 

 X 68. 



29. A section through the posterior mesodermal connection (ms.co.) of the 

 belly-stalk. The Trager shows the villi in the process of formation. X 30. 



