406 AN INTRODUCTION TO ZOOLOGY 



curved slit, the myocoel ; as a matter of fact it is more poten- 

 tial than actual in the chick. The somite, then, is a more or less 

 clearly marked oblong mass of cells having a definite arrangement. 

 It is joined on to the parietal mesoderm, between whose somatic 

 and splanchnic layers there is by this time a fairly large coelomic 

 space, by a narrow strip of cells variously known as the intermediate 

 cell mass, the somitic stalk or nephrotome. The last name is applied 

 to indicate that this small portion is destined in the main to give 

 rise to the excretory system, but a part of it also contributes to the 

 formation of the mesenchyme. 



It is now necessary to return again to the central nervous 

 system, whose beginning we have already noted in the medullary 

 plate. Practically the whole of this structure in front of the primi- 

 tive knot is destined to take part in the formation of the brain and 

 is consequently sometimes distinguished as the brain plate. The 

 medullary folds gradually get higher and higher, and the medullary 

 groove in consequence deeper and also nai rower. Finally the folds 

 turn inwards and grow towards the middle line, where they meet 

 and fuse, thus converting the plate into the neural tube and the 

 groove into the neural canal. They meet first at a point a little way 

 back from the front end of the head fold in a region that will give 

 rise later to the mid-brain. Before the actual fusion has taken 

 place, however, it will be noticed that the folds at the anterior end 

 are much deeper and more strongly marked than posteriorly, conse- 

 quently the neural tube when formed is markedly larger in front 

 than behind. This swelling forecasts the division between the 

 brain and spinal cord regions of the central nervous system. The 

 two are not sharply demarked and pass insensibly into one another. 

 We have already noted that the first four pairs of somites are included 

 in the head of the adult, and therefore the hinder limit of the fourth 

 pair may be taken as marking the end of the brain and beginning 

 of the spinal cord. 



From the very first the brain is much larger than the cord, and 

 even in a chick of 33 hours' incubation (i.e. with twelve pairs of som- 

 ites), it occupies more than half the total length of the neural area, 

 a precocious development apparently necessitated by the relatively 

 large size of the brain when fully grown. In the embryo of 24 hours, 

 before the fusion of the folds, another specialisation has made its 

 appearance, and that is that the front end of the folds is marked off 

 by a distinct constriction, while the hinder parts of the folds simply 

 decrease uniformly as they pass backwards. The first point of 

 fusion is behind this constriction, and from it the union extends 

 forwards slowly, leaving a small aperture, the neuropore, for some 

 while right at its front end. The lips of this opening when they fuse 



