CHAPTER Xn 



ORGAN DEVELOPMENT IN VERTEBRATES 



IT is clearly impossible, iii a book of this size, to do more than sketch in 

 very broad outline the extremely complex processes by which the details 

 of the adult anatomy come into being. There are, moreover, already many 

 excellent accounts of them; for more extended comparative treatments, 

 Dalcq and Gerard's revision of Brachet (1935), and particularly Nelsen's 

 recent work (1953) can be recommended. 



In the present chapter, an attempt has been made to single out those 

 aspects of organ development in which new advances have recently been 

 made, or which are particularly important in connection with various 

 general principles of development; and two organs, the hmbs and the 

 kidneys, which provide very interesting illustrations of a number of 

 points, have been dealt with in rather fuller detail. 



I. The general form of the embryo 



At the end of gastrulation, the amphibian embryo — often known at 

 this stage as a neurula — is still a solid, approximately spherical object. The 

 neural system runs along its dorsal meridian, at first in the form of an open 

 neural plate, which however rapidly closes up into a neural groove and 

 fmally a tube. It also stretches considerably in the anterior-posterior direc- 

 tion, continuing that process of elongation which we have seen to be 

 characteristic of the dorsal region through gastrulation. At an early stage 

 in this stretching, the head and tail curl round the opposite ends of the 

 egg, which has now become somewhat ovoid, so that the embryo seems 

 to be on the point of biting its tail ; but as the elongation becomes still 

 greater, particularly in the more posterior regions, the dorsal axis straigh- 

 tens out again and the creature begins to assume the form of a tadpole, 

 with a long thin tail stretching out behind its body. During all this time, 

 there has been a tubular gut, formed directly by the invaginating meso- 

 derm and endoderm, and opening out to the exterior through the blasto- 

 pore, which lies at its posterior end in the region where the definitive anus 

 will eventually be. At first the tube has a floor and walls of endoderm and 

 a roof of mesoderm, but the lateral edges of the endoderm soon grow in 

 from each side to meet in the midline and from then on the gut-tube is 

 hned entirely by endoderm; we shall see that this situation is not so easily 

 brought about in the flat blastoderm of the birds. The wall of the 



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