1902.] NATURAL SCIENCES OF PHILADELPHIA. 91 



pharjTix. Fig 4a 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 now by four invagina- 

 tions of its walls. The two that have been spoken of as the " lat- 

 eral invaginations" (i.v.), pushing into the thick side walls in a 

 dorso-median direction, are not very different from what we saw 

 in fig. 2b. The other two push toward each other from the roof 

 and floor of the gland (fig. 4a, m. I. ) until they meet and fuse, 

 thus separating the thyroid into two distinct lateral halves. At a 

 later stage of development mesoblast cells press in between these 

 halves to form a thin mesoblast ic partition. At this stage the 

 mesoblast 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 '* Pseudobrauchial- 

 rinne ' ' ) makes its appearance as a groove in the floor of the 

 pharynx, extending anteriorly a short distance from the opening of 

 the gland (fig. 46, v. e.g.). At this stage no cilia could be seen, 

 and the groove was much wider than it was at later stages of de- 

 velopment. The cavity of the gland, in the region represented in 

 fig. 46, 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. 4a. The other changes noticed at this 

 stage are the closing of the slit-like opening into the pharynx until 

 only a small circular canal is left (no section through this opening 

 is represented), the thinning of what we shall hereafter call, after 

 Dohrn, the "cover" cells (" Decklamelle") (fig. 46, d.L), and 

 the thickening and deeper invagination of the "gland" cells 

 (" Druseulamelle " ) (fig. 46, g.L). The part of the gland pos- 

 terior to what we may call now the ' ' duct ' ' remains about as in 

 the preceding stage (fig. 4c). 



The larvae of the next stage were about 9 mm. long, and showed 

 all the outward characteristics of the normal Ammoccetes. An 

 outline drawing, from life, of the side of the head is shown in fig. 

 5a. Being drawn under the microscope with a camera, the rela- 

 tive size and position of the thyroid are accurately shown. The 

 gland is seen to extend from the first to the fifth gill-arch, and to 

 be closely wedged in between the pharynx above and the body 

 wall below. The upwardly curved ends did not show in the living 

 animal, except as indicated by the upward bending of the longitu- 



