GENERAL DIRECTION OF DEVELOPMENT. lOlS 



The tergale zone is occupied by a new formation, which appears to be the 

 continuation of the primitive trace, but is in reality altogether distinct. This is 

 a furrow formed by a longitudinal excavation in the ectoderm, and which com- 

 mences the trace of the central nervous system ; it is named the medullary 

 groove, or dorsal furrow. At the bottom of this groove a dark line betrays 

 the presence of the chorda dorsalis, or notochord — a provisional skeletal stalk 

 that sustains the medullary groove. 



The presence of the medullary groove leads to the division of the embryo 

 into two perfectly distinct zones — the spinal zone, corresponding with the 

 medullary groove, and the marginal zone comprising all the other part to the 

 borders of the embryo. These facts become very intelhgible in a transverse 

 section through the middle of the medullary groove (Fig. 551). It is seen how 

 the latter is formed by the thickening and median inflection of the ectoderm, 



Fig. 551. 



TRANSVERSE SECTION OP A BLASTODERM AFTER THE FORMATION OF THE MEDULLART GROOVE 

 AND THE NOTOCHORD. 



A, Ectoderm ; B, mesoderm ; c, endoderm : mc, medullary groove ; mf, medullary fold ; ch, noto- 

 chord. The figure represents the left half of the section. 



which resembles a V widely open, the two branches of the V constituting the 

 medullary folds (Fig. 550). By their inflection and union they ultimately form 

 a closed tube — the neural canal, perhaps better designated as the neuraxis 

 (Fig. 552). 



The endoderm does not offer any other modification, but the mesoderm 

 undergoes transformations of the highest interest. The interposition of the 

 chorda dorsalis divides it into two symmetrical moieties. The thickened parts, 

 which, in the two moieties, are subjacent to the corresponding medullary fold, 

 form the lateral layers (or plates). Beyond, the mesoderm shows, in certain 

 embryos at this time, a very marked indication of a decisive event in the evolu- 

 tion of the embryo and the realization of the vertebral type ; but it is more 

 convenient to study this in a more advanced embryo. 



In transverse sections, the mesoderm can be seen undergoing cleavage into 

 two layers (Fig. 552). The space between these — and which is really a fissure 

 of no importance — grows to a great extent ; this is the pleuro-peritoneal fissurSy 

 which at a later period becomes the pleuro-peritoneal cavity. 



The two layers resulting from the splitting of the mesoderm, perform very 

 difl'erent functions. The superficial layer, which furnishes materials for the 

 parieties of the body — including the dermis of the skin — has been named the 

 musculo-cutaneous layer ; the deeper one is designated the fibro-intestinal layeTy 

 which sufficiently indicates the direction of its approaching evolution. 



In their ulterior modifications, the two layers remain closely allied to the 

 ectoderm and endoderm ; and by this association they constitute formations 

 akeady complex, which it is necessary to note and to qualify. The musculo- 



