140 WALTER HEAPE. 



due to the comparatively early development of the nervous 

 system. During the early part of Stage d there is a consider- 

 able mass of primitive hypoblast along the axial line of the 

 embryo, but the rapidly forming medullary groove pressing on 

 to this mass before it has become formed into a rod capable of 

 resisting much pressure, causes it to bulge inwards and thus 

 flattens out its cells, administering an effective check to the 

 development of the organ. Such a check occurs during Stages 

 D and E. Subsequently the thickening of the lateral meso- 

 blast plates and the consequent depression of the lateral hypo- 

 blast, removes the strain from the axial cells and admits of the 

 isolation of the slender rod or band which exists for the 

 greater portion of the length of an embryo Mole at the close 

 of Stage J. 



The Mesoblast. 



At the close of Stage d the mesoblast in front of the 

 primitive streak is in the form of two lateral plates which are 

 connected together across the middle line by means of a mass 

 of undifferentiated hypoblast, except during a short space 

 where they are separated by the deep portion of the medullary 

 groove. 



At the periphery these mesoblastic plates are split into two 

 layers, an upper somatic and a lower splanchnic layer, along 

 the whole of their extent posterior to the cephalic plate. The 

 split is entirely peripheral, however, and does not extend into 

 the embryonic area {vide former paper No. 8) . 



The Mesoblastic Somites and the Body Cavity. — During 

 Stage E the splitting of the mesoblast extends further forwards, 

 and also inwards towards the medullary groove. I have never 

 been able completely to satisfy myself that this splitting ever 

 extends to the innermost portion of the mesoblastic plates ; 

 hut, as I have before explained, the small size of the embryo 

 and the dense compact nature of the middle layer renders it 

 exceedingly difficult accurately to determine such a point. 



The nearest approach to a continuous split of the mesoblast 

 from the axial portion to the periphery which I have seen, is 



