DEVELOPMENT OF THE FROG. 551 



at this spot, the blastopore. The embryo elongates slightly, 

 but the mass of yolk-laden cells which lie on the floor of the 

 gut prevents the body acquiring at once the fish-like shape. 

 The blastopore as usual marks the posterior region of the 

 body. 



The processes which follow are already in outline familiar 

 to the student. Along the mid-dorsal line an epiblastic 

 neural plate is differentiated. The edges become raised 

 into the neural folds ; these approach one another, and, 

 fusing together, form the medullary or neural canal. At the 

 posterior end this communicates with the archenteron for a 

 time by the neurenteric canal. Internally, a differentiation of 

 hypoblast forms the noto- 

 chord along the mid-dorsal 

 line of the archenteron. 

 At each side of this lie 

 masses of mesoblast which $5 XJSXtvM^ZSlffiKfey arch - 



have been split off from 

 the hypoblast. Each of 

 these divides into the 5 

 primitive segments (proto- 

 vertebra?) above, and the 



unsegmented lateral plates F , Gi 237 _Gastrula stage of newt.— After 



below. The lateral plates Hertwig. 



Split into tWO layers, the ep., Epiblast;Ae./>., hypoblast ;arc^.,archen- 

 1 i_ • • ■ t teron ; v., yolk-cells; s.c. , segmentation 



splanchnic or inner invest- cav i ty . 

 ing the gut, the somatic 



or outer layer being applied to the epiblast; the space 

 between the two layers is the body cavity. The body now 

 becomes distinctly divided into regions, the eyes bud out 

 from the brain, a rudiment of the external gills appears, and 

 the larva, still within its gelatinous case, exhibits peculiar 

 lashing movements of the tail. 



Eventually, about a fortnight after the eggs are laid, 

 it escapes from the surrounding jelly and swims freely 

 in the water. At this stage and for some time the 

 ectoderm is ciliated. There is a cloacal opening, but 

 the mouth is not yet more than a dimple. A glandular 

 crescent, often misnamed a sucker, lies on the under sur- 

 face of the head, and secretes a sticky slime, by means of 

 which the tadpole attaches itself to foreign objects. The 



