AXIAL RUDIMENTS OF THE EMBRYO. 



691 



This primitive axis constitutes in some measure the centre of sub- 

 sequent changes of development. It consists mainly of a thickening 

 produced by the accumulation of blastodermic cells. 



1.— INTRA-EMBRYONIC PHENOMENA OF DEVELOPMENT. 



Axial Utidiment of the Embryo. — Cerebro-spinal Axis. — The 



genetic changes which lead to the first formation of the rudiments of 

 the embryo may be briefly sketched as follows : — 



The longitudinal thickening of the blastoderm, which forms the 

 primitive trace, belongs at first chiefly to the upper layer or epiblast, 

 but soon extends to the central part of the middle layer or mesoblast. 

 The hypoblast takes no share in its production. 



The elongated plate or thickening is very soon separated towards the 

 cephalic end of the primitive trace by a median groove or linear 

 depression into two lateral plates, which, thickening to some extent, 

 rise into ridges and thus constitute the laminaa dorsales, or dorsal 

 ridges. The groove deepening, and the ridges becoming more 

 elevated, there is then formed a canal, and by the further eleva- 

 tion of the ridges, their approach to each other, and their final coal- 

 escence in the middle line, the canal is gradually closed in along the 

 dorsal line. The part of the upper layer which has undergone this 

 inflection and enclosure acquires considerable increased thickness, but 

 still a cavity remains in its interior. The part where it was closed 

 dorsally now becomes separated fi-om the upper layer or epiblast with 

 which it was originally continuous, and the latter passes subsequently 

 free and entire across the dorsal line. 



Fig. 501. — E.MBRi'0 OF THE DoG SEEN FROM ABOVE, Fig. 501. 



WITH A PORTION OP THE BLASTODERM ATTACHED. 



Tlie medullary canal is not yet closed, but shows the 

 dilatation at the cephalic extremity with a jjartial division 

 into the three primary cerebral vesicles ; the posterior 

 extremity shows a rhomboidal enlargement. The cephalic 

 fold crosses below the middle cerebral vesicle. Six pri- 

 mordial vertebral divisions ure visible ; so, the upper 

 division of the blastoderm ; sp, the lower division, where 

 they have been cut away from the peripheral x^arts. 



This canal is wider at the cephalic ex- 

 tremity in which the rudiment of the brain is 

 situated, it is of uniform diameter in the suc- 

 ceeding or middle part, and at the caudal 

 extremity remains open for a time, but is closed 

 in at a later period like the rest. 



The rudiment of the great nervous centre 

 arises in a thickening of the central portion 

 of the enclosed epiblast which is originally 

 continuous with the rest of the upper layer ; 

 but this part which forms the brain and spinal 

 marrow exhibits considerable thickening at an 



early stage, thus constituting what by some have been called the medul- 

 lary plates, while the canal is still open, and subsequently folded round 

 dorsally and closed in the form of a medullary tube, within which 



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