FORMATION OF EMBRYONIC ECTODERM 



29 



veutrally. This would correspond to the extension round the wall of the blastocyst that occurs 

 in lower mammals. 2. The layer bends round in front and grows backwards to close at the 

 posterior end. 3. The cavity is formed by a splitting among the budded-off entoderm cells so 

 that they are separated into two lamella?. None of Hubrecht's figures of this stage directly 

 favour this last possibility, but it has been suggested that the yolk-sac in the human ovum 

 may be formed out of a solid mass of cells in this way. 



Formation of the embryonic ectoderm : bilaminar blastoderm. 



Though the primitive entoderm is already formed, the stage of a bilaminar blastoderm 

 is not yet reached. The 

 formative cell-mass is still a 

 rounded knob, continuous 

 with, but to be distin- 

 guished from, the tropho- 



phase 



place 



blast. In the next 

 a splitting takes 



among the cells, so as to 

 form a cavity (amnio- 

 embryonic cavity, fig. 40) in 

 the heart of the knob. 

 The cells forming the floor 

 of the cavity arrange them- 

 selves in a columnar 

 manner, and form a plate, 

 which is the embryonic 

 ectoderm. This plate is 

 at first necessarily concave 

 owing to the invagination 

 of the formative cell-mass. 

 The fate of the cells form- 

 ing the roof of the cavity 

 differs in Tarsius and the 

 higher Primates. In Tar- 

 sius, as the plate increases 

 in size it becomes flattened out ; the primitive invagination (inward projection) is 

 undone, and the embryonic ectoderm comes to lie free on the surface of the 

 blastocyst by the disappearance, due to retraction or otherwise, of the cells forming 

 the roof of the amnio- embryonic cavity. At a considerably later period of 

 development, when the embryo has been laid down, the cavity is as it were again 



FIG. 40. HYPOTHETICAL STAGE or HUMAN BLASTOCYST 



(T. H. Bryce.) 



tr., trophoblast ; am., amnio-embryonic cavity ; 

 ams., amnion-stalk ; ent., entoderm. 



FIG. 41. SECTION OF PART OF THE BLASTODEEMIC VESICLE OF THE RABBIT AT six DAYS. 



(From E. Van Beueden.) 



a, trophoblast (Rauber's layer) ; b, formative ectoderm ; c, entoderm. 



restored by the formation of folds which meet over the embryo and form the 

 definitive amnion, as will be explained later. 



In man and the apes this early stage is as yet unknown ; but the later phases 

 are most adequately explained by assuming that the roof of the cavity persists, 

 that the primary invagination is not at this stage undone, and that the formative 

 ectoderm never comes to lie free on the surface of the blastocyst. The cavity in 

 the formative cell-mass thus becomes the definitive amniotic cavity, which is 



