398 OUTLINES OF CHORDATE DEVELOPMENT 



'among the cells of the head process. In other forms, such as 

 the sheep, pig, or hedgehog, and in man, the notochordal canal 

 is simply a vertical perforation through the embryonic layers, 

 connecting the cavity of the endodermal vesicle directly with 

 the outside (amnionic cavity) (Figs. 160, A; 161, Z>). And in 

 still other forms the canal may never quite perforate the layers 

 of the shield (mole) ; or it may be reduced to a simple groove 

 on the lower surface of the shield (rabbit, Fig. 160, B). 



Sections through the notochordal canal reveal an arrange- 

 ment of the germ layers which is practically that found around 

 the more typical blastopore of the yolk-filled egg. In this 

 region the layers are continuous with one another and the primi- 

 tive groove may be deepened into a sort of " primitive pit" 

 (compare chick). Altogether then we may recognize in the 

 primitive streak of the Mammal the essential equivalent of the 

 similarly named structure in the Sauropsida. The notochor- 

 dal canal therefore becomes the modified equivalent of the 

 blastoporal remains. 



When the notochordal canal has the form of a simple perfora- 

 tion it may therefore correctly be termed the neurenteric canal, 

 marking the posterior limit of the embryonic rudiment. The 

 primitive streak, representing the modified and fused blasto- 

 poral lips, is here, as in the chick, a region from which the 

 structures of the embryo are derived and differentiated in the 

 antero-posterior direction, again much as in the chick. 



All details regarding the formation of the embryonic layers 

 and organs are to be omitted from the present account. It 

 must suffice to say. that a typical medullary plate is formed, 

 medullary folds appear and fuse, forming a neural tube (Figs. 

 158, B; 159, B). A typical notochord is. differentiated, run- 

 ning forward from the primitive knot. The mesodermal layer 

 rapidly extends laterally from the axial region, finally passing 

 widely beyond the embryonic region and taking a very impor- 

 tant part in the development of the extra-embryonic structures. 

 The mesodermal sheet is very early split into somatic and 

 splanchnic layers by the appearance of a coelomic cavity; both 

 embryonic and extra-embryonic. 



