EMBRYOLOGY 



225 



The Early Embryo. — Several of the chief systems of organs are laid 

 down at a very early time. The general arrangement of these systems 

 must be pointed out before any of their parts are traced into later stages. 

 One of the first changes visible externally is the appearance of two promi- 

 nent ridges, close together, along the dorsal side of the future embryo. 

 These extend lengthwise and are roughly parallel except at the anterior 

 end where they diverge from one another. In a cross-section of the frog, 

 for example, these ridges appear as in Fig. 180. They are the neural 

 folds, the beginning of the central nervous system. Where these folds are 

 near one another the spinal cord develops; the divergent folds in front 

 form the brain. These ridges approach one another and fuse along their 

 upper surfaces, cutting off a tube beneath the ectoderm. In longitudinal 

 vertical section at this time, the nervous system appears as in Fig. 181. 



e.c nd 



on 



Fig. 181. — Longitudinal section of the early embryo of a frog, diagrammatic, art, anus; 

 br, brain; c, ccelom; ec, ectoderm; en, endoderm; int, intestine; li, liver; ms, mesoderm; 

 nd, notochord; sp, spinal cord; st, stomach. 



Beneath the nervous system a cylindrical rod of cells, the notochord, 

 has been separated off from the endoderm beneath. Around it later is 

 formed the back-bone. 



The digestive tract has been present, as the archenteron, ever since 

 gastrulation took place. At the stage which is described in the preceding 

 paragraph it is usually enlarged in front and narrowed behind. These 

 parts correspond roughly, in the frog, to the stomach and intestine. 

 Posteriorly the intestine opens to the outside through the anus which in 

 some animals is the same opening as the blastopore, but in others a 

 passage secondarily produced. In many animals the anus first appears at 

 a much later stage. 



As indicated above, and in Fig. 180, the mesoderm is early divided 

 into two layers, one applied to the inside of the ectoderm, the other cover- 

 ing the endoderm. The peritoneum which occupies approximately 



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