134 THE THIRD DAY. [CHAP 



According to Miiller (Ueber die Fn he irk clung der Srhilddriise. Jenaische 

 Zeitschrift, 1871) who has recently studied its development with great care, the 

 thyroid body arises on the third day as an involution from the hypoblast of the 

 throat opposite the point of origin of the second arterial arch. This involution 

 becomes by the fourth day a solid mass of cells, and by the fifth ceases to be 

 connected with the epithelium of the throat, becoming at the same time bilobed. 

 By the seventh day it has travelled somewhat backwards, and the two lobes 

 have completely separated from each other. By the ninth day the whole is 

 invested by a capsule of connective tissue, which sends in septa dividing it into 

 a number of lobes or solid masses of cells, and by the sixteenth day it is a paired 

 body composed of a number of follicles, each with a ' membrana propria,' and 

 separated from each other by septa of connective tissue, much as in the adult. 



22. Coincidently with the appearance of these several 

 rudiments of important organs in the more or less modified 

 splanchnopleure-folds, the solid trunk of the embryo is 

 undergoing marked changes. 



When we compare a transverse section taken through 

 say the middle of the trunk at the end of the third day 

 (Fig. 44), with a similar one of the second day (Fig. 20), or 

 even the commencement of the third day (Fig. 41), we 

 are struck with the great increase of depth (from dorsal 

 to ventral surface) in proportion to breadth. This is partly 

 due to the slope of the side walls of the body having become 

 much steeper as a direct result of the rapidly progressing 

 folding off of the embryo from the yolk-sac. But it is also 

 brought about by the great changes both of shape and 

 structure which are taking place in the proto vertebrae as 

 well as by the development of a mass of tissue between the 

 notochord and the hypoblast of the alimentary canal. 



23. The protovertebrae on the second day, as seen in a 

 transverse section, Fig. 20, P.v., are somewhat quadrilateral 

 in form but broader than high. Each at that time consists 

 of a somewhat thick cortex of radiating rather granular 

 columnar cells, enclosing a small kernel of spherical more 

 transparent cells. 



Bemak and after him Kblliker have described the centre of the proto- 

 vertebra? as being simply fluid without structural elements. His explicitly 

 denies this in the case of the protovertebrae of the neck, and it seems probable 

 that the centre is in all cases really occupied by transparent spherical cells. 



Towards the end of the second and the beginning of 

 the third day, the central cells increase rapidly in number 

 (Fig. 41), and towards the end of the latter day (Fig. 44), as 

 it were lift up and push out the columnar cortex above and 

 at the outer side. In this way the portions forming the 



