ORGAN DEVELOPMENT IN VERTEBRATES 25I 



amphibian gut is at first rather thin on the dorsal side, but very thick on 

 the ventral, where the cells are still swollen by the large stores of yolk. 

 From the exterior, it appears as though little is happening on the ventral 

 side while the tail is growing out, but beneath the skin the first blood cells 

 are forming, and the mesoderm is beginning to form a pulsating tube, the 

 rudiment of the heart. 



The chick embryo, when it reaches the neural plate stage, is not, to 

 speak crudely, a solid lump like the amphibian, but is instead a flat circular 

 plate, on which the embryo proper is linear in form, the remainder of the 

 plate being no part of the final body but concerned only with the absorp- 

 tion and digestion of the yolk and with the respiratory exchanges between 

 the embryonic blood and the outside air. One of the most striking features 

 of the embryo proper is the very great difference in the stage of develop- 

 ment reached by the anterior and posterior parts. In the Amphibia the 

 whole neural plate folds up more or less simultaneously to form a neural 

 tube, which stretches from the anterior right to the posterior end, where 

 there is only a small region of *tail-bud' at which new additions of neural 

 tissue and mesoderm continue to be formed. In the cliick on the other 

 hand, at a time when the anterior neural plate has folded up into a com- 

 pletely closed tube in the head region, it is stiU only a shallow groove at 

 the level of about the tenth somite, while posterior to this there is still a 

 remnant of the primitive streak at which mesoderm is being formed and 

 the primary organiser is active. Thus although differentiation progresses 

 roughly from anterior to posterior in both forms, the differences are much 

 more marked in the chick than in the frog or newt. 



Fairly soon after the closure of the neural tube in the anterior region of 

 the chick, one can see the beginning of a process by which the embryo 

 proper eventually becomes separated from the remainder of the blasto- 

 derm. Along a crescent-shaped line in front of the head, the blastoderm 

 is tucked downwards and backwards under the neural tube. The fold so 

 formed is known as the head fold. As it gets deeper and is pushed further 

 backwards, the head region is gradually left isolated on a projection above 

 it. At a considerably later stage, when the embryo has some thirty or more 

 pairs of somites, a similar fold (the tail fold) cuts under the posterior end 

 of the embryo ; and eventually these two folds each progress so far to- 

 wards the centre of the body that the whole embryo is left attached to the 

 non-embryonic blastoderm only by a narrow stalk (Fig. 12. i). 



As the blastoderm is tucked downwards in the head fold, a pocket lined 

 with endoderm appears on the under side of the head. This is the first 

 rudiment of the gut to appear, and is known as the foregut, to distinguish 

 it from the hindgut which appears later in comiection with the tail fold. 



