608 FRANCIS M. BALDWIN 



II. OBSERVATIONS 



1. Thymus bodies 



A. Ambly stoma larvae, 8 mm. long. At the 8 mm. stage (figs. 

 4 and 5) the mouth is not open, the place where it is to form being 

 occupied by an extensive oral plate, in which the exact limits of 

 entoderm and ectoderm can be recognized only uncertainly and 

 with difficulty (Kingsley and Thyng '04, and Johnston '10). The 

 pharyngeal lumen {ph.) extends from behind forward into this 

 plate. The pharyngeal region is a flattened tube throughout. 

 It gives off on either side, the visceral pouches — the future gill 

 clefts — which have not yet broken through. The most anterior 

 of these pouches, the hyomandibular, lies about the level of the 

 eye in the series on which these observations are based. The 

 outer surface of its entodermal walls does not quite reach the 

 external ectoderm of the head, while below and in front of it is a 

 mass of mesoderm, the ventral prolongation of the third head 

 somite. A conical projection, consisting of a few cells at the 

 posterior dorsal angle of this pouch is the anlage of the first 

 thymus body. The projection is some distance from the ecto- 

 derm, between which and the anlage are cells, some of which 

 stain more darkly than the surrounding cells. These darker 

 cells are apparently those which Driiner has interpreted as the 

 ectodermal contribution to this thymus body, concerning which 

 more will be said below. 



This first thymus anlage (fig. 6), lies about mid- way between 

 the eye and the ear. Dorsal to it is the developing facial {VII) 

 ganglion, while the internal carotid artery {i.e.) lies medial to it 

 and sends a small twig (the quadrato-mandibular artery) for- 

 ward and ventrally to supply the anterior region of the body, the 

 latter passing just in front of the thymus anlage. The anlage on 

 the left is seen in three, and the one on the right, in four successive 

 sections (each 10 micra), but it is difficult to limit its extent 

 exactly. 



The second pouch (first branchial) is much like the hyomandib- 

 ular in its general features. Externally it extends in the longi- 



