216 The Anatomy of a 12-mm. Pig 
an elongated contact with the ectoderm, since its course is parallel 
with the corresponding part of the second cleft. De Meuron, 86, p. 
104, states that in no class of mammals other than vertebrates can 
anything be found comparable with this ventral ccecum. 
The closing plate of the third cleft lies very near a deep ectodermal 
pocket which opens externally by a clear-cut nearly round hole, and 
extends internally along the course of the tenth nerve, ending in an 
epithelial proliferation. A very slender tube from the pharynx runs 
toward this mass. The pocket, gland and tube mark the course of the 
fourth cleft. 
From the base of the entodermal part of the fourth cleft there is 
another pocket tending to pass around in front of the trachea. De 
Meuron considered this to be a rudimentary cleft. It may, however, 
be a ventral branch of the fourth cleft comparable with that of the 
third. Both of these ventral processes occupy parallel positions in the 
embryo. 
In a 12-mm. pig, the cervical sinus is mainly the ectodermal part of 
the fourth cleft as above described. Its rounded opening has not yet 
been concealed by the opercular extension of the hyoid arch. The clos- 
ing plate of the third cleft is becoming involved in the orifice of the 
sinus anteriorly. Posteriorly the outlet of the sinus is in contact with 
the ganghon nodosum forming Froriep’s epibranchial organ, 85. The 
epithelial prolferation shown in Plate Il is below the level of the 
ganglion. 
The thyroid gland is represented in the 12-mm. pig by its somewhat 
branched median anlage in the second arch, and by the ventral arms of 
the fourth cleft. The latter, known also as the post-branchial hodies 
or lateral thyroids, encircle the trachea, become detached from the 
pharynx, and, in the higher mammals only, connect with the median 
thyroid. 
The thymus is derived mainly from the ventral arm of the third 
pouch which also encircles the trachea, approaching its mate from the 
opposite side. In the 12-mm. pig there is an epithelial mass connected 
with the entoderm of the third arch and with the adjacent ectoderm. 
In the 14-mm. pig, this tissue has apparently fused with that at the 
tip of the cervical sinus, and the resulting mass is also in contact 
with the ganglion nodosum. Froriep, 85, p. 32, found that his study 
of the epibranchial organ was complicated inasmuch as the cell mass, 
connected on one side with the ganglion of the tenth nerve, was united 
on the other with the anlage of the thymus. Such a condition appears 
ina 1?v-mm. pig. De Meuron, 86, p. 76, found that in sheep the fourth 
