694 GENERAL PHENOMENA OF EMBRYONIC DEVELOPMENT. 



the embryo extends into the space between the amnion and the other 

 developed membranes of the ovum. 



Inflection of the Walls of the Body of the Embryo. — The first 

 rudiments of the embryo, as before described, lie prone and flat on the 

 surface of the yolk, consisting almost entirely in thickenings, with 

 some incurvations, of certain parts of the blastoderm. In the forma- 

 tion of these parts the two upper layers, epiblasfc and mesoblast, are 

 alone concerned, and the hypoblast takes no part in them, but passes 

 thin and flat across the space occupied by the embryonic rudiments. 



In the further progress of development a great change of form is 

 now produced by the downward inflection of the whole three layers of 

 the blastoderm, in consequence of which the embryo rises, as it were, 

 out of the plane of the rest of this membrane, and begins to be notched 

 off" from its peripheral parts. The first of these folds, termed crplmlic, 

 (fig. 504) takes place at the extremity of the embryo which contains the 

 rudiment of cranium representing the head, and precedes by a con- 

 siderable interval the other folds. A similar downward fold subsequently 

 follows at the caudal extremity, and there is also between the cephalic 

 and caudal folds a simultaneous depression of the layers of the blasto- 

 derm in lateral folds, so that the embryo takes in some measure the form 

 of an inverted boat, with its keel upwards, and its hollow side opening* 

 towards the yolk cavity, and the fore part being, as it were, partially 

 covered in by the deck of the cephalic fold. Thus are produced the 

 downward ventral or visceral plates which form the side walls of the 

 head and trunk ; and at a later period, by the increased constriction 

 or convergence of the folds round the place of communication between 

 the embryo and the peripheral parts of the blastoderm, there is formed 

 the umbilicus (see figs. 510 and 512). 



Ym. 505. 



Fig. 505. — Transyersk Section through the 

 EMBRro-CnicK before and some time after 



THE CliOSURE OF THE MEDULLARY CanAL, 

 TO SHOW THE UPWARD AND DOWNWARD IN- 

 FLECTIONS OF THE Blastoderm (after llemak). 



A, At the end of the first day. 1, notochord ; 

 2, primitive groove in the medullary canal ; 3, 

 edge of the dorsal lamina ; 4, corneous layer or 

 epiblast ; 5, mesoblast divided in its inner part ; 

 6, hypoljlast or epithelial layer ; 7, section of 

 protovertebral plate. 



B. On the thii-d day in the lumbar region. 

 1, notochord in its sheath ; 2, medullary canal 

 now closed in ; 3, section of the medullary sub- 

 stance of the spinal cord ; 4, corneous layer ; 

 5, somatopleure of the mesoblast ; 5', splanchno- 

 pleure (one figure is placed in the pleuro- 

 peritoueal cavity); 6, hyjioblast layer in the intes- 

 tine and spreading over the yolk ; 4x5, part of 

 the fold of the amnion formed by epiblast and 

 somatopleure. 



The fundamental steps, therefore, in the development of the verte- 

 brate embryo result in the formation in the axial part, or head and 

 trunk of the body, of two cavities, of which one is situated above and 

 the othor below the notochordal axis ; the upper constituting the 

 cranio-vertebral canal, and containing the rudiment of the cerebro- 



