. The biology of the frog . orsal lip of blastopore; BP, ventral lip of blastopore; EE, epidermiclayer of ectoderm ; EN, inner or nervous layer of ectoderm; //.hypo-blast or entoderm; T, mesenteron or gastrula cavity; Y, yolk plug.(After Marshall.) is completed there appears a third germ layer, the mesodermor mesoblast, between the other two. The mesoderm ap-pears all around the blastopore, and as this opening closesmainly from in front backward, the two masses of meso-derm on either side are brought near each other in the mid-dorsal line. The free ventral edges of the masses or sheets THE DEV
. The biology of the frog . orsal lip of blastopore; BP, ventral lip of blastopore; EE, epidermiclayer of ectoderm ; EN, inner or nervous layer of ectoderm; //.hypo-blast or entoderm; T, mesenteron or gastrula cavity; Y, yolk plug.(After Marshall.) is completed there appears a third germ layer, the mesodermor mesoblast, between the other two. The mesoderm ap-pears all around the blastopore, and as this opening closesmainly from in front backward, the two masses of meso-derm on either side are brought near each other in the mid-dorsal line. The free ventral edges of the masses or sheets THE DEVELOPMENT OF THE FROG 97 of mesoderm extend ventrally until they meet below andcome to surround the archenteron, except for a short spacealong the dorsal side. The sheets of mesoderm soon be-come split into an inner or splanchnic layer, which lies nextto the archenteron, and an outer, parietal, or somatic layer,which lies next to the ectoderm. The space between thesetwo layers of mesoderm is the be-inning of the cos lorn, or. FlG. 21. —Transverse section through the middle of a frogs embryo. CH,notochord; E, ectoderm ; M, mesoderm ; AG, neural groove; NP,neural plate ; T, mesenteron ; Y, yolk cells. (After Marshall.) body cavity. It is at first small, but as development proceeds,it widens out more and more. The cells just above the mid-dorsal wall of the archenteronform a thickening which soon becomes marked off sharplyfrom the mesodermic layers on either side and the wall ofthe archenteron below. This thickening is the beginningof the notochord, a structure forming the beginning of the ,,s THE BIOLOGY OF THE FROG vertebral column, and occurring in the embryo, when notalso present in the adults, of all vertebi-te animals. It NO
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