GENERAL DIRECTION OF DEVELOPMENT. 101$ 



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

 continuation of the primitive trace, but is in reality altof^ether distinct. This ia 

 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 medullar// 

 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 zme comprising all the other part to the 

 borders of the embryo. These facts become very intelligible 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. 



en 



TRAKSVERSE SECTION OF A BLASTODERM AFTER THE FORMATION OF THK MF.DULLARY 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 Y 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 latc'ral 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 fissure, 

 which at a later period becomes the pleiiro-peritonecd cavity. 



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

 different 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 fihro-intestinal layer, 

 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 thi-; association they constitute formations 

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



