FORMATION OF MESOBLASTIC SOMITES. 



37 



prowvcrtelm, on the supposition (now known to be erroneous) that they are 

 the rudiments of the future vertebras ; they are more appropriately termed the 

 mesoblast ic somite*. 



M.c. 



So. 



\ 



Fig. 39. TRANSVERSE SECTIOK THROUGH THE DORSAL REGION OP AN EMBRYO CHICK OP 45 HOURS. 



(FromBalfour.) 



A, epiblast ; C, hypoblast ; Me, medullary canal ; Pv, protovertebra or mesoblastic somite ; Wd, 

 intermediate cell-mass in which the Wolffian duct is becoming formed ; So, somatopleure ; Sp, 

 splanchnopleure ; pp, pleuro-peritoneal cavity (ccelom) ; op, inner margin of area opaca ; w. thickened 

 hypoblast of area opaca ; ao, left primitive aorta ; v, blood vessels ; ck, notochord. 



In Amphioxus (figs. 28, 29), the protovertebrse are formed in common with the body cavity, and 

 are successively separated off from before backwards from the ccelomic fold as hollow cuboid 

 somites, each of which extends upwards around the neural canal and downwards along the 

 sides of the alimentary canal, and subsequently divides into a dorsal or paraxial part which 

 forms the protovertebrse, and a ventral part which forms the lateral mesoblast. At first the 

 hollow somites communicate individually with the alimentary cavity, but they become shut 

 off from this long before the division which has just been mentioned. The ventral segments 

 run together eventually, to form a continuous serous cavity. In sections of bird or mammalian 

 embryos (fig. 39), the protovertebrse, although on the whole compact masses of meso- 

 blast, yet often show a tendency to have their central cells loosely arranged, so as to give the 

 appearance of an irregular cleft in their interior, and sometimes a definite cavity is formed in 

 them, which may even be continuous with the ccelomic cleft in the lateral mesoblast. 



Protovertebrse begin to be marked off in the paraxial mesoblast, a short distance 

 from the anterior end of the neural groove, in what will eventually become the 

 cervical region of the embryo. They are produced in succession from before back- 

 wards, one or two only being at first visible on either side, and others being gradually 

 added as the embryo grows in length, until a large number may at length be counted, 

 extending from immediately behind the cephalic region to the region of the primitive 

 streak. 



Cerebral vesicles. Meanwhile a change of importance has taken place in 

 connection with the anterior end of the neural groove, which has become enlarged, 

 and soon exhibits a succession of highly characteristic median dilatations, separated 

 from one another by slight constrictions (fig. 34). These dilatations, at a later 

 stage, after they have become roofed in, along with the rest of the neural groove, 

 are known as the cerebral vesicles. There is at first a simple enlargement, and 

 behind this two others form in succession, ?o that the primary resides are three in 



