66 WALTER HE APE. 



come into closer relations with one another. Fig. 42 represents 

 the latter condition in a somewhat similar emhryo. 



Anterior to the medullary groove the lateral hypoblast and 

 mesoblast are not yet separated, and a continuous mass of 

 undifferentiated cells underlies the epiblast plate. 



Behind the groove the primitive streak occasions changes 

 identical with those already described (p. 55). 



The mesoblast throughout the embryo projects beyond the 

 limits of the area, and is there split into somatic and splanchnic 

 layers. The relations of the neurenteric canal I will describe in 

 detail in another place (on p. 57) ; for this specimen I will 

 only say that at the junction of the primitive streak and me- 

 dullary groove a deep pit is formed by the involution of the 

 epiblast in the middle line ; the pit is widely open above, but 

 enters a mass of mesoblast below, and is there, as far as I could 

 see, entirely obliterated. 



The groove now deepens, forcing the notochordal cells under- 

 lying it further downwards, and in this way the latter, while 

 remaining connected with the hypoblast, becomes separated 

 from the lateral masses of mesoblast. Such relations are shown 

 in fig. 44, which is a transverse section through the medullary 

 groove of an embryo slightly older than fig. 13, taken from the 

 same relative position as the section in fig. 43. 



This is, however, the deepest portion of the medullary 

 groove, and only in this section and those immediately on each 

 side of it do the relations hold which are here figured. Both 

 anteriorly and posteriorly the groove is more shallow, and the 

 axial hypoblast is continuous with both lateral mesoblast and 

 hypoblast. 



The structure of the remainder of the embryo is identical 

 with that described above for fig. 13. 



The amnion in this embryo is completely formed over the 

 hind end of the primitive streak, although not so far advanced 

 at the front end of the area. 



In describing the next embryo I will give an account of the 

 structure of the neurenteric canal. 



The arrangement of the layers at the front end of the primi- 



