40 GENERAL EMBRYOLOGY. 
aperture, the boundaries of which have already been described, and posteriorly it is 
in direct continuity with the pharynx. 
The division of the common nasal chamber into two parts commences before its 
separation from the mouth is completed, and it is brought about by the development 
of a septum which is continuous anteriorly with the fused nasal lamine, and which 
grows downwards and backwards from the mesial part of the under aspect of the 
fronto-nasal process. This septum fuses below with the conjoined margins of the 
palatal ledges of the maxillary processes, and a vertical plate of cartilage soon 
develops in its interior, which is continuous above with the cartilaginous base of the 
eranium (basi-cranial axis). A portion of this septal cartilage remains in the adult 
as the septal cartilage of the nose, and the remainder is more or less completely 
replaced by the vertical plate of the ethmoid bone and by the vomer. The lateral 
ain wall of each nasal chamber is formed, 
in the lower part of its extent, by the 
maxillary process of the mandibular arch, 
in which the superior maxillary, malar, 
and palate bones, and possibly the inter- 
nal pterygoid plate, are developed, and 
in the upper part by the outer boundary 
of the original nasal pit, which now 
forms only the upper part of the nasal 
cavity. In this upper section of the 
outer wall an outgrowth of the basi- 
cranial axis projects downwards, and is 
developed into the lateral mass of the 
ethmoid bone; probably it also takes 
part in the formation of the inferior 
turbinal bone. 
The fusion of the three segments of 
the palate commences anteriorly at the 
eighth week by the union of the maxil- 
lary and globular processes; it passes 
Fic, 32.—VerTICAL SEoTION THROUGH Heap or backwards and is completed by the 
aT EMBRYO. fusion of the posterior parts of the 
Showing the formation of the two parts of the pituitary palatal ledges of the maxillary processes, 
pod Maan) about the tenth week. To the non- 
(Ectoderm is represented in black, entoderm in blue, 
andinicsadenntinlred®) completion of this fusion the various 
At. Atlas. P. Pineal body. cases of hare-lip and cleft palate are due. 
Ax, Axis. PR. Cerebral hemisphere. 
Ca as Pralan, Bes ae Hoh pial cop he Organ of Jacobson.—The organs 
THe kT Pt2, Buccal part of pituitary Of Jacobson are rudimentary structures in 
HB. Hind-brain. body. man. ‘They lie in the lower and anterior 
MB. Mid-brain. SG. Spinal ganglion. part of the nasal septum, one upon each 
N. Part of nasal cavity. T. Tongue. 
The THalamencephaon: side. They are developed as small diverti- 
cula which grow backwards and upwards 
in the substance of the septum, and their points of commencement are situated 
immediately above the intermaxillary segment of the palate. Each diverticulum is 
partially surrounded, on its inner side, by a cartilaginous capsule, it ends blindly behind, 
and it opens anteriorly close to the floor of the nose in the region of Stenson’s foramen— 
a small aperture left between the premaxillary and maxillary sections of the bony palate. 
The Pituitary Body.—The pituitary body is formed partly from the floor of 
the first primary cerebral vesicle, and partly from the roof of the stomatodeal 
space. The stomatodeal portion appears as a small pouch, Rathke’s pouch, which 
grows dorsally into the base of the head immediately in front of the dorsal end of 
the bucco-pharyngeal membrane and the anterior end of the notochord and behind 
the fore-brain. It is lined by ectoderm, and soon becomes a conical vesicle which 
lies beneath the base of the fore-brain. Its orifice of communication with the 
stomatodeal space is gradually constricted until the lumen disappears, and then 
for a time the vesicle is connected with the surface by a solid cord of ectodermal 
