OUTLINES OF THE DEVELOPMENT OF THE TUATAEA. 19 



pleure strongly downwards. Traced backwards this cavity 

 appears to divide into two irregular fissures (fig. 9), lying one 

 on either side beneath the head-fold. 



The development of a large coelomic cavity at this stage^ and 

 in this situation, seems very remarkable. That it is not in 

 any way due to accident seems to be proved by the marked 

 differentiation of the hypoblast cells beneath it, which acquire 

 their characteristic short columnar form earlier in this parti- 

 cular region than anywhere else. Comparison with later stages 

 seems to me to prove pretty conclusively that it is due to a 

 very early development of that part of the coelom which will 

 later on form the pericardium, being carried backwards into 

 the body of the embryo Avith the lower limb of the head-fold 

 (compare figs. 24, 50, P. C). 



Fig. 9 represents a section passing through the head-fold, 

 which lies quite freely above the blastoderm. In the blasto- 

 derm beneath the head-fold are seen the backward prolonga- 

 tions of the coelomic space above mentioned, and the epiblast 

 has becotne flattened. In the embryo itself the epiblast on 

 the dorsal surface forms a medullary plate, in the middle of 

 which lies the medullary groove {31. G.). The alimentary 

 canal (Al. C.) is completely enclosed in this region, forming 

 a wide space lined by slightly flattened hypoblast cells, not 

 at all sharply marked off, especially below, from the niesoblast 

 cells. The latter, for the most part, form a loose network of 

 irregular cells lying between epiblast and hypoblast, but denser 

 and apparently undergoing more rapid division on the ventral 

 surface. 



Fig. 10 represents a section just behind the head-fold, show- 

 ing the alimentary canal still widely open below. The epi- 

 blast on the dorsal surface on each side of the medullary 

 groove is widely separated from the underlying mesoblast, but 

 the floor of the medullary groove itself remains in close con- 

 tact with it. The mesoblast and hypoblast of die embryo are 

 clearly seen to be here formed primarily by infolding of the 

 original lower layer of the blastoderm before it has become 

 clearly differentiated into two layers. It seems likely, how- 



