DEVELOPMENT OF THE FROG. 461 



and more numerous than those below. Sections show a 

 small segmentation cavity within. The upper layer cells 

 gradually spread round and enclose the lower. But at one 

 point, where upper and lower cells meet, an invagination of 

 upper layer cells takes place. This invagination represents 

 the formation of a gastrula; the cavity formed is the em- 

 bryonic gut or archenteron, its roof is formed from invag- 

 inated cells, its floor from yolk-laden cells ; the opening or 

 blastopore of the invagination corresponds in position to the 

 anus. The cells, several rows deep, which surround the 

 outside, and were originally those of the upper hemisphere, 

 are epiblastic ; the invaginated layer establishes the hypoblast ; 

 between the two, especially about the lips of the blastopore, 

 the mesoblast soon appears. This stage, in which the three 

 germinal layers are established, is reached about the fifth or 

 sixth day after fertilisation. 



Thereafter, definite signs of the embryo begin to appear. 

 Along the mid-dorsal line of the epiblast, a neural plate is 

 differentiated, whose edges become raised and folded over, 

 forming the medullary groove, which becomes the medullary 

 or neural canal. A communication between this neural canal 

 above and the archenteron below, is retained in the region 

 of the blastopore. It is known as the neurenteric canal. 

 Internally, the archenteron becomes larger and more definite, 

 and its extension causes the segmentation cavity to disappear. 

 Along the mid-dorsal line of the archenteron, a differentiation 

 of hypoblast establishes the notochord. On each side of 

 this lie segments of mesoblast, which used to be called 

 protovertebrae, while inferiorly the mesoblast has divided into 

 two layers, an outer somatic applied to the epiblast, an inner 

 splanchnic investing the archenteron, the space between 

 them being the body-cavity. About a fortnight after hatching, 

 the form of the body is beginning to be distinct. Soon 

 afterwards, the embryo begins to move its tail within the 

 surrounding jelly, external gills grow out from the two hind- 

 most of the four visceral arches, the mouth appears as an 

 oval blind pit, two suckers lie just behind it, the eyes have 

 been budded out from the brain and are showing themselves 

 externally, the surrounding mucus disintegrates, and the 

 embryo jerks itself away from it. 



