OUTLINES OF THR DEVELOPMENT OF THE TUATABA. 25 



embryo. On tho ventral surface of the embryo the alimentary 

 canal is outlined by two parallel grooves, united in front by a 

 deep crescentic transverse fold which marks the limit to which 

 the anterior enclosure of the alimentary canal has as yet 

 extended (fig. 16, F. Sp.). 



Fig. 17 represents the same embryo seen from above as a 

 transparent object after staining and clearing. 



Figs. 18 — 20 represent three corresponding views of embryo 

 64. The sub-embryonal membrane not having been so com- 

 pletely removed in this case, the hinder end of the embryo 

 could hardly be made out at all as an opaque object. 



Fig. 20 is especially worthy of note^ as it shows very clearly 

 the spreading of the lateral wings of mesoblast [L.JV.) derived 

 from the primitive streak. It will be seen that they spread 

 outwards and forwards as two irregular sheets^ following 

 mainly the line of junction between the areapellucida and area 

 opaca. Reference to transverse sections (figs. 30, 31) show 

 that these sheets [L.W.) lie in the floor of the developing 

 pleuro- peritoneal space which separates the serous envelope 

 from the future yolk-sac. They lie, in fact, in the yolk-sac 

 itself, where they undoubtedly give rise to a portion at any 

 rate of the vitelline vessels (compare figs. 20 and 58). 



Turning now to the more minute structure of these embryos, 

 the following features seem worthy of note. 



The enclosed portion of the alimentary canal extends further 

 back than in the preceding stage, and is considerably narrowed, 

 while the hypoblastic cells forming its wall have assumed their 

 characteristic short columnar shape (compare figs. 9 and 23). 

 The change from flattened to columnar in the shape of the hypo- 

 blast cells extends also for some distance behind the limit of 

 closure (except in the mid-dorsal line), but gradually ceases as 

 we approach the primitive streak (figs. 25 — 33). 



The noiochord has made its appearance in the middle line be- 

 neath the medullary groove. It appears to me quite clear that 

 it has been formed by separation of a solid rod of cells from the 

 sheet of mesoblast which has already been noticed as growing 

 forward from the primitive streak, and which, it will be re- 



