256 PRINCIPLES OF EMBRYOLOGY 



hindbrain plays an important part in calling forth the thickenings, but so 

 does the local mesoderm, and a fully normal ear can only develop when 

 both components induce together in a harmonious way. 



There are in reality whole sequences of 'secondary' organisers, acting 

 one after the other; perhaps they should be classed as secondary, tertiary, 

 quaternary organisers and so on, but it would be difficult to do this in any 

 very sensible way. For example, after the eye-cup has induced the lens, 

 the whole complex then induces the skin lying above it to become trans- 

 parent and to differentiate into the cornea, and the mesoderm which 

 clothes it to become the sclerotic coats of the eyeball. Again the original 

 ectodermal placode of the ear induces the mesoderm to form the other 

 parts of the ear structure. 



Within its ectodermal covering, the head of course contains not only 

 the brain but also an infilling of mesoderm and some endoderm. The 

 latter forms the pharynx, or anterior part of the gut, and will be discussed 

 below in connection with the latter. It remains here to say something 

 about the mesoderm. This tissue has a double origin. The greater part of 

 it is formed by invagination through the blastopore or primitive streak, 

 and this is the part to which we have so far paid most attention ; but some 

 mesoderm is also formed from cells which follow quite a different path. 

 When the two neural folds finally come together and fuse to form the 

 neural tube, some of the cells at the two fusing edges break loose and 

 move down between the tube and the overlying ectoderm. These have 

 been given a variety of names by various authors ; sometimes they are 

 alluded to as 'mesectoderm', a word which is also used for the epiblast of a 

 blastoderm before the mesoderm has invaginated and thus become separ- 

 ated from the ectoderm; a somewhat better name is 'ecto-mesoderm', 

 which is not so ambiguous, but probably it is simplest and best to speak 

 of tliis second contribution to the mesoderm as the 'neural crest material', 

 a phrase which clearly describes what is meant (Fig. 12.7, p. 266). 



In most of the trunk, the neural crest material is scanty compared 

 with the whole bulk of the mesoderm; it eventually forms the pigmented 

 cells of the skin and contributes to the spinal ganglia. In the head it plays 

 a much more important part, and forms large masses of tissue, which 

 develop not only into some of the cranial nerves but also give rise to many 

 parts of the cartilaginous skeleton (Horstadius and Selman 1942, de Beer 

 1947)- 



3. The gut: anterior portion 



In the Amphibia, it looks at first sight as though the formation of the 

 gut is extremely simple, since a complete closed tube is formed during the 



