632 



REPRODUCTION. 



FIG. 187. 



cerebro-spinal axis ; and the canal itself becomes, in the adult, the cen- 

 tral canal of the spinal cord, with its continuations in the encephalon, 

 namely, the fourth ventricle, aqueduct of Sylvius, and third ventricle. 

 The external portion of the ectoderm, remaining 1 outside the medullary 

 canal and covering the surface of the embryo, becomes the epidermic 

 layer of the general integument. 



The coalescence of the dorsal plates, by which the medullary groovo 

 is converted into a canal, does not take place at the same time through- 



out. It is first completed in the 

 middle portion of what will after- 

 ward be the head ; the anterior part 

 of the encephalon, and the cervical 

 and dorsal portions, remaining open 

 until a later period. Their final clos- 

 ure proceeds in a general direction 

 from before backward, occupying 

 successive portions as development 

 goes on, and reaching at last the pos- 

 terior extremity . 



The dorsal plates, as the imme- 

 diate precursors of the cerebro-spinal 

 axis, are the first distinct indication 

 of a permanent embryonic organ. 

 Their relation to the primitive trace 

 is not very well defined, and it is 

 doubtful whether they are especially 

 connected with its formation. They 

 first appear in advance of its anterior 

 extremity ; and, although the medul- 

 lary groove between them corre- 



* ' ltS * 1 lonitudinal 



EUBIMEKTAKTEMBHYOOFTHECHICK.atthe 



thirtieth hour of incubation. p v . Proto- direction with the median furrow of 



vertebrae. Dp. Dorsal plates. Mq. Medullary ,v > , 



groove, p,. Primitive trace. (Koiiiker.) the Punitive trace, the two are not 



uniformly continuous, but, according 



to Koiiiker,* are often laterally displaced, the one falling a little to the 

 right, the other to the left. The medullary groove is moreover consid- 

 erably wider than the primitive furrow ; and, while the cephalic region 

 of the embryo, as well as its cervical and dorsal portions, grow very 

 rapidly with the progress of incubation, the primitive trace remains 

 confined to the caudal extremity. Its greatest length, about the thir- 

 tieth hour of incubation, is rather less than two millimetres ; it begins 

 to diminish perceptibly from the fortieth to the forty-second hour, and 

 at the end of the second day has almost disappeared. By this time 

 the medullary canal is closed for nearly the whole length of the cere- 

 bro-spinal axis. 



* Embryologie. Paris, 1879, pp. 109, 141. 



