The Pharyngeal Pouches in the Mammalia 227 



The cat of 31 mm. (Fig. 68, CGI.) shows the carotid gland as an 

 ovoid body, located near the antero-dorsal angle of the lateral wing of 

 the thyroid. It shows its follicular structure very clearly. It has lost 

 its connection with the cervical cord of the thymus, the latter terminat- 

 ing opposite the ventral border of the thyroid. 



In a 20-day rabbit (No. 172, Harvard series) the carotid gland shows 

 its typical form and structure. It lies outside of the antero-dorsal angle 

 of the lateral wing of the thyroid and is connected by the cervical cord 

 with the thoracic thymus. 



In the 21-day rabbit the gland occupies a depression in the outer sur- 

 face of the lateral wing of the thyroid. It is now entirely unconnected 

 with the cervical cord, the latter lying at a much lower level. 



(c"') The Sinus Prascervicalis and its Eelation to the Thymus. 



It will be recalled that in a 6.2 mm. cat (Figs. 59-60) the sinus 

 prfficervicalis (S.Pc.) forms an approximately funnel-shaped depression. 

 The inner posterior portion corresponding to the stem of the funnel 

 forms a deep pit, which extends diagonally inwards and backwards and 

 terminates in a sharp edge, which at its ventral extremity is in contact 

 with the lateral process of the fourth pouch. The outer relatively wide 

 vestibule forms an approximately triangular depression, surrounded on 

 all sides by the overhanging prominences of the adjacent parts — ante- 

 riorly by the hyoid arch, posteriorly by the anterior cervical region and 

 ventrally by the pericardial prominence. Dorsally it becomes shallow 

 and there blends with the side of the head without any perceptible break. 

 At its ventro-anterior angle it is continued into the pericardio-cervical 

 groove (Fig. 60). The inner wall of the sinus is formed by two low 

 prominences representing the third and fourth pharyngeal arches. 



In the 9.7 mJii. cat the sinus prsecervicalis (Fig. 61, S.Pc.) is deeper 

 and narrower than in the stage just described — a difference due to the 

 continued outgrowth and approximation of the adjacent hyoid and 

 anterior cervical regions. That part of the third arch which imme- 

 diately adjoins the hyoid region is rotated outwards with the latter and 

 thus comes to face obliquely baclcwards. In this way the outer opening 

 of the deeper part comes to lie on a level with the third phar5rngeal 

 groove (Ph.G.3). We shall henceforth designate this deeper part of 

 the sinus the fundus prgecervicalis (F.Pc), a term applied to it by 

 Kastschenko. 



The opening of the fundus is triangular, narrow above, wider below 

 (see right side of figure). At its dorsal apex the third pharyngeal arch 



