FOEMATION OF THE CENTRAL NERVOUS SYSTEM. 



Sadler, If the husband is younger than the wife, there are as many boys as girls ; if both are 

 of the same age, there are 1029 boys to 1000 girls ; if the husband is older, 1057 boys to 1000 

 girls. In insects, food has a most important influence. Pfliiger's investigations on frogs show 

 that all external conditions during development are without effect on the determination of the 

 sex, so that the latter would seem to be determined before impregnation. 



451. FORMATION OF THE CENTRAL NERVOUS SYSTEM. Fore-brain. At each side 



of the fore-brain, or anterior cerebral vesicle, which is covered externally by epiblast and in- 

 ternally by the ependyma, there 



grows out a large stalked hollow 7 /^ h 



vesicle, the rudiment of the cere- 

 bral hemispheres. The relatively 

 wide opening in the stalk, or com- 

 munication, ultimately becomes 

 very small, and is the foramen of 

 Monro. The middle part between 

 the two cerebral vesicles remains 

 small, and is the 'tween or inter- 

 brain with the 3rd ventricle in its 

 interior. It elongates at the second 

 month towards the base of the brain 

 as a funnel-shaped projection, to 

 form the tuber cine re um with the 

 infundibulum. The thalami optici, 

 projecting and enlarging from the 

 sides of the 3rd ventricle, narrow th 

 the foramen of Monro to a semi- 

 lunar slit. At the base of the brain 

 are formed, in the 2nd month, the 

 corpora albican tia, at the 3rd the 

 chiasma ; while within the 3rd ven- 

 tricle the commissures are formed. 

 The hypophysis, belonging to the 

 mid-brain, is a diverticulum of the Fig. 689. 



nasal mucous membrane, extending Transverse section of the brain of an embryo sheep 2*7 cm. 

 through the base of the skull to- long; x 10. a, cartilage of orbito-sphenoid ; ^peduncu- 

 lar fibres ; ch, optic chiasm ; /, median cerebral fissure ; 

 h, cerebral hemispheres, with a convolution upon their 

 inner wall, projecting into the lateral ventricle, I ; m, 

 foramen of Monro ; o, optic nerve ; p, pharynx ; pi, 

 lateral plexus ; s, termination of the median fissure, 

 which forms the roof of the third ventricle ; sa, body 

 of the anterior sphenoid ; st, corpus striatum ; t, third 

 ventricle ; th, anterior deep portion of the optic thalamus 

 (Kolliker). 



wards the hollow infundibulum, 

 which grows to meet it (fig. 505, 

 T). There is, as it were, a tend- 

 ency to the union of the cavity of 

 the fore-gut with the medullary 

 tube. In the amphioxus {Kowal- 

 eivsky), goose (Gasser), and lizard 

 (Strahl) the medullary tube com- 

 municated originally with the hind- 

 gut by the canalis myeloentericus. 

 The choroid plexus, which grows into the ventricles of the hemispheres through the foramen of 

 Monro, is a vascular development of the ependyma. At the 4th month, the conarium (pineal 

 gland) is formed, and at this time the corpora quadrigemina cover the hemispheres. The corpora 

 striata begin to be developed in the cerebral (lateral) ventricle at the 2nd month, while the cornu 

 ammonis is formed at the 4th month. [The external walls and floor of the primitively simple 

 central hemispheres become much thickened, the thickenings in the floor constitute the corpora 

 striata, which protrude into the lateral ventricles, their position being indicated on the 

 surface of the brain by the Sylvian fissure. As they extend backwards, they become con- 

 nected with the optic thalami (fig. 689, st, th). The corpora striata are connected together by 

 the anterior commissure. From the inner wall of each hemisphere, there grow into each 

 lateral ventricle two projections ; the upper one forms the hippocampus major or cornu 

 ammonis (fig. 689, h), while the lower one becomes folded, remains thin, receives numerous 

 blood-vessels from the falx cerebri, and forms the choroid plexus (fig. 689, pi).] At the 3rd 

 month the Sylvian fissure is formed, and the basis of the island of Reil. The permanent 

 cerebral convolutions are formed from the 7th month onwards. 



The mid-brain, or middle cerebral vesicle, is gradually covered over by the backward growth 

 of the hemispheres ; its cavity forms the aqueduct of Sylvius (fig. 690). Depressions appear 

 on the surface of the vesicle to divide it into four, the corpora quadrigemina, in birds into 

 two, the corpora bigemina (fig. 690, bg), the longitudinal depression being formed at the 3rd, 

 and the transverse one at the 7th month. The cerebral peduncle is formed by a thickening 

 in the base of this vesicle. 



