182 THE THIED DAT. [CHAP. 



with the epithelium of the throat, becoming at the same time 

 bUobed. By the seventh day it has travelled somewhat back- 

 wards, 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 its two 

 lobes are composed of a number of folhcles, each with a 'mem- 

 brana propria,' and separated from each other by septa of con- 

 nective tissue, much as in the adult ^. 



The spleen. Although the spleen cannot be reckoned 

 amongst the glands of the alimentary tract its development may 

 conveniently be dealt with here. It is formed shortly after the 

 first appearance of the pancreas, as a thickening of the me- 

 sentery of the stomach (mesogastrium) and is therefore entirely 

 a mesoblastic structure. The mass of mesoblast which forms 

 the spleen becomes early separated b}"- a groove on the one side 

 from the pancreas and on the other from the mesentery. Some 

 of its cells become elongated, and send out processes which, 

 uniting with like processes from other cells, form the trabecular 

 system. From the remainder of the tissue are derived the cells 

 of the spleen pulp, which frequently contain more than one 

 nucleus. Especial accumulations of these take place at a later 

 l)eriod to form the so-called Malpighian corpuscles of the spleen. 



The AUantois. We have already had occasion to 

 point out that the allantois is essentially a diverticvilum 

 of the alimentary tract into which it opens immediately 

 in front of the anus. Its walls are formed of vascular 

 splanchnic mesoblast, within which is a lining of hypo- 

 blast. It becomes a conspicuous object on the third 

 day of incubation, but its first development takes place 

 at an earlier period, and is intimately connected with 

 the formation of the posterior section of the gut. 



At the time of the folding in of the hinder end of 



1 Miiller Ueber die Entwickehtng dcr Schilddrilse. Jenaische 

 Zeitschrift, 1871. 



