76 THE EARLY DEVELOPMENT OF MAMMALS. 



formed into the ventral fissure, figure 40, Fiss., filled with mesenchyma or, in the 

 adult, by vascularized connective tissue. Further, the inner surfaces of the dorsal 

 zone meet and unite, thus obliterating the dorsal portion of the original central 

 canal, and forming out of the fused ependymal layers the permanent posterior 

 partition, figure 40, Part. The central canal of the adult, figure 40, Can, corresponds, 

 therefore, to the ventral part only of the original canal. 



Plakodes. 



Plakodes are small circumscribed thickenings of the ectoderm, which contribute 

 to the development of the olfactory, visual, and auditory organs. There are accord- 

 ingly three pairs of them. They resemble one another very closely in original ap- 

 pearance and their early history. A plakode, figure 41, Plk, is a rounded area of 



-. 



EC. Plk. Seg. 



Mes. Ent. Endo. Ao. 



FIG. 41. CHICK EMBRYO OF THIRTEEN SEGMENTS. SAGITTAL SERIES 1452, SECTION 52. 



Ao, Aorta. EC, Ectoderm. Endo, Endothelium. Ent, Entoderm. Mes, Mesoderm. Plk, Auditory plakode. 



Seg, Mesodermic somite. X 100 diams. 



syncytial ectoderm of small dimensions, several times as thick as the surround- 

 ing ectoderm, with which it merges by a rapid transition in the diameter of the 

 layer. Soon after its appearance it becomes invaginated. By preserving this con- 

 dition the olfactory plakode forms the olfactory pit. In other cases the invagina- 

 tion deepens, then closes over, and the vesicle thus formed separates from the over- 

 lying ectoderm. The vesicle formed by the visual plakode becomes the lens of the 

 eye. The vesicle formed by the auditory plakode becomes the otocyst. The ulti- 

 mate development of these organs is considered in the three sections next following. 



The Nasal Pits and Olfactory Nerves. 



The olfactory plakodes arise as a symmetrical pair of thickenings underneath 

 the fore-brain. They soon become invaginated, forming two shallow depressions just 

 in front" of the mouth, as is well shown in figure 165. As development progresses, 

 the depressions deepen and remain lined throughout by the thickened ectoderm 

 (Fig. 194, No). The orifices may be temporarily closed by the coalescence of the 

 epithelium. The olfactory pits acquire a secondary opening into the oral cavity 

 (Fig. 219). By the expansion of this cavity, the nasal chamber proper of the adult 



