192 COMPARATIVE ANATOMY 



the widely-open mouth is the blastopore. The layer of larger 

 cells lining the hollow of the cup is the primitive hypoblast ; 

 the outer layer is the epiblast. 



Such an embryonic form is known as a gastrula. It soon 

 loses its hemispherical shape and becomes elongated, the 

 blastopore at the same time being reduced to a small orifice 

 situated at one extremity of the now ovoid embryo. The 

 manner in which the narrowing of the blastopore is effected 

 has been the subject of much controversy. It would appear 

 that the walls of the widely-open cup, depicted in fig. 47, K, 

 are folded together, with the result that the blastopore is 

 narrowed to the dimensions shown in L. This folding is 

 accompanied by a multiplication of cells at the lips of the 

 blastopore, so that the orifice is not slit-like but circular. 

 The posterior end of the embryo is indicated by the blastopore, 

 and one side — the dorsal side of the future animal — is flattened. 



About this time the epiblastic cells develop cilia, by means 

 of which the embryo rotates within the vitelline membrane. 

 Thus far, development has led to the formation of a two- 

 layered gastrula, not differing in any essential feature from 

 the gastrula of many invertebrate animals. In the succeeding 

 stages the gastrula undergoes important changes, leading to 

 the formation of the mesoblastic somites and of characteristic 

 vertebrate organs, the dorsal nerve tube and the notochord. 



The embryo becomes elongated in an antero - posterior 

 direction, and the cells of the flattened dorsal surface are some- 

 what modified to form an elongated median tract known as the 

 neural plate. This plate sinks down below the surface, and 

 the cells of the adjoining epiblast begin to grow over it, 

 and eventually they meet and unite in the middle line above 

 it. During this process the sides of the neural plate are bent 

 upwards, so that it forms a gutter, roofed in above by the 

 epiblast which has grown over it. Eventually, by further 

 upgrowth, and folding in of its sides, the gutter is converted 

 into a tube, the neural tube or central nervous system of 

 the adult. While the dorsal epiblast is growing over the 

 neural plate, the epiblast at the hps of the blastopore — 

 particularly that of the ventral lip — grows over the blastopore, 

 and, being continuous in front with the dorsal epiblast, it 

 shuts off" the blastopore from the exterior, but roofs over a 

 passage leading round the posterior extremity of the embryo 



Digitized by Microsoft® 



