ORGAN DEVELOPMENT IN VERTEBRATES 253 



side of the endoderm and no further folding would be needed to bring 

 them together; we should in fact have arrived at the amphibian condition. 

 The reverse of this can actually be achieved experimentally. Nieuwkoop 

 (1946) has slit an amphibian neurula up the ventral midline, removed the 

 endoderm and flattened out the mesoderm and ectoderm on a plate of 

 stretched silk, so that it is forced into a sort of 'blastodermic' configura- 

 tion. It then develops two heart rudiments, one on each side, which 

 fail to fuse so as to form a single heart, since the embryo is not pro- 

 vided with the foregut-forming mechanism which brings this about in 

 the chick. 



In the stage immediately after its formation, the heart of the chick 

 embryo continues to have an important influence on the general form of 

 the body. In the first place, it becomes extremely large. This is necessary 

 because it has to pump blood not only through the embryo itself, but for 

 very much longer distances through the blood vessels which run into the 

 non-embryonic blastoderm. It becomes so disproportionate to the body 

 proper that it bulges out to one side. At the same time, the whole body 

 rotates, so as to lie with its left side down against the yolk; and the head also 

 arches round so that the anterior end of the brain is bent down towards the 

 chest, with the heart protruding between them. It is the development of 

 the heart and its associated structures which is the cause of most of these 

 foldings and rotations, and if it is removed, the main part of the neural 

 system remains straight, only the anterior tip of the brain bending down- 

 wards. 



2. The development of the head 



In both the amphibian and the bird, the neural plate is wider in the 

 anterior than in the posterior. This is very clear in the newt, in which the 

 whole plate is marked out on the surface of the egg at the time the blasto- 

 pore is closing; it is definitely pear-shaped. As the edges of this area fold 

 together to form the neural tube, the future brain, developing from the 

 anterior end, has from the beginning a wider lumen than the hinder parts 

 of the nerve cord. At first the tube remains open at its anterior tip, the 

 small hole connecting the interior of the tube with the outside being 

 known as the anterior neuropore; it closes gradually during the later 

 development of the brain. 



The comparatively slight initial difference in the width of the brain and 

 of the main nerve tube soon becomes greatly increased by the appearance 

 of swellings in the former. At first there are three, which form the primary 

 brain vesicles which give rise to the forebrain, midbrain and hindbrain. 

 Fairly soon the first and third swellings each become differentiated into 



