-11- 



into two distinct lateral parts, from the anterior end about 

 half way to the opening into the pharynx. Fig, 4 a is a section 

 through the extreme posterior edge of the partition, showing 

 the invagination of dorsal and ventral walls. The once simple 

 tube is complicated nov,r by four invaginations of its walls. 

 The two that have been spoken of as the "lateral invaginations" 

 (i.v.), pushing into the thick side v/alls in a dorso-median di- 

 rection, are not very different from •«ti»4"~*e -eaw in Fig. 2 b. 

 The other tw^o push towards each other from the roof and floor of 

 the gland (Fig. 4 a, m.l.) until they meet and fuse, thus sepa- 

 rating the thyroid into two distinct lateral halves. At a later 

 stage of development mesoblast cells press in betv/eon these halves 

 to form a thin mesoblastic partition. At this stage the meso- 

 blast is composed still of closely-packed, rounded cells, and 

 contains a few scattered yolk granules. It is at this time also 

 that the first trace of the ciliated grooves (the "Pseudo- 

 branchialrinne") makes its appearance as a groove in the floor 

 of the pharynx, extending anteriorly a short distance from the 

 opening of the gland (Fig. 4 b, v.c.g.). At this stage no cilia 

 could be seen, and the groove was much wider than it was at later 

 stages of development. The cavity of the gland, in the region 

 represented in Fig, 4 b, still retains, in cross section, its 

 resemblance to the letter T. This section is anterior to the 

 duct, but posterior to the median lamella represented in Fig. 4 a. 

 The other changes noticed at this stage are t- the closing of the 



