THE EMBRYO. 



1173 



hind-brain (Fig. 701). Subsequently the anterior and posterior vesicles each 

 become constricted into two, while the middle one remains undivided. It will thus 

 be seen that at the anterior extremity of the medullary canal there are five dilata- 

 tions, separated from each other by constrictions, through which, however they 

 freely communicate with each other. These five vesicles are the five fundamental 

 livisions of the adult brain, and are named from before backward: prosencephalon 

 thalamencephalon, mesencephalon, epencephalon, and metencephalon (Figs. 723 



en 



FIG. 720. Vertical section of the head in early embryos of the rabbit. Magnified. (From Mihalkovics.) 

 A. From an embryo five millimetres long. B. From an embryo six millimetres long. d. Vertical section 

 of the anterior end of the notochord and pituitary body, etc., from an embryo sixteen millimetres long. In A, 

 the faucial opening is still closed. In B, it is formed, c. Anterior cerebral vesicle, me. Mesocerebrum. mo. 

 Medulla oblongata. co. Corneous layer, m. Medullary layer, if. Infundibulum. am. Amuion. spe. Spheno- 

 ethmqidal. be. Central (dorsum sellae), and spo. spheno-occipital parts of the basis-cranii. h. Heart. /. 

 Anterior extremity of primitive alimentary canal and opening (later) of the fauces, i. Cephalic portion of 

 primitive intestine, tha. Thalamus. p'. Closed opening or the involuted part of the pituitary body (py). ch. 

 Notochord. ph. Pharynx. 



and 725). They are at first fairly uniform in size and shape, but soon begin to 

 grow at different rates and assume different forms. The changes are most 

 marked in the first vesicle. 



The first secondary vessel (prosencephalon) sends out two hollow protrusions, 

 one on either side, from the forepart of its lateral surface ; these grow rapidly and 

 spread out and extend forward, laterally, and backward over the sides of the 

 first and second vesicles, forming large cavities, which become the lateral 

 ventricles (Fig. 725, G). From each, three prolongations take place: one, 

 forward and outward; a second, backward and inward; and a third, at first 

 backward, outward, and downward, and then forward and inward ; these form 

 the horns of the lateral ventricles. These prolongations far exceed in size the 

 original vesicle from which they sprung, which does not increase to any great 

 extent. It remains as the anterior part of the third ventricle (Fig. 725, A), and 



