130 Elementary Zoology. 



The next important change to occur is the welding together 

 of the lowest layer of shells to form a continuous sheet, to 

 which the name hypoblast is applied. Between the hypoblast 

 and the epiblast are a few cells which take part in the formation 

 of the third layer of the embryo, the mesoblast. Soon a median 

 opacity appears along the embryo, the primitive streak. 

 This, when examined by means of transverse sections, is seen 

 to be due to a thick band of cells, which is marked superficially 

 by a groove. The thickened band of cells is produced by a 

 rapid growth of the epiblast cells, which results in a sheet of 

 tissue, as is shown in the accompanying figure (Fig. 67). 



This is the main portion of the mesoblast, the rest being 

 produced fro.m the proliferation of hypoblast cells ; and some of 



pa ps M 



I / / 



Fig. 67 — Section through primitive strealc of Chick at 20th hour. (After Marshall.) 

 pSj primitive streak ; PG, the groove upon it ; e, ep 'blast ; M, mesoblast : h, hypoblast. 



its cells are persistent lower-layer cells, already referred to as 

 left after the separation of the hypoblast. 



Among the cells proliferated off from the hypoblast to form 

 the mesoblast, a number form themselves into a densish rod of 

 tissue, running along the body of the embryo. This rod of tissue 

 is the notoehord, which is formed, it will be observed, from the 

 hypoblast. The notoehord is the first part of the skeleton to 

 appear. Directly after the appearance of the notoehord, the 

 first beginnings of the central nervous system are laid down. 

 In front of the primitive streak is a thickened layer of epiblast, 

 the neural plate. A groove, the neural groove, is formed 

 along its surface, which becomes, later, closed to form a canal, 

 the neural tube. The closure of the groove into a tube com- 

 mences with the head end and spreads backward. The cavity 

 of the central nervous system persists in the adult as the 

 ventricles of the brain and the canal of the spinal cord. It 



