DEVELOPMENT OF THE EMBRYO. 1009 



it subsequently presents, especially as regards the relative proportions of its 

 different segments. The encephalon, at about the sixth week, is seen as a 

 series of vesicles arranged in a line with each other (Fig. 275); of which those 

 that represent the Cerebrum (6) are the smallest, whilst that which represents 

 the Cerebellum (d) is the largest. The latter (or Epencephalon), as in Fishes, 

 is single, covering the fourth ventricle on the dorsal surface of the Medulla 

 Oblongata. Anterior to this is the single vesicle (a) of the Corpora Quadrige- 

 mina (or Mesencephalon), from which the optic nerve chiefly arises ; this has in 

 its interior a cavity, the ventricle of Sylvius, which is persistent in the adult 

 Bird, though obliterated in the adult Mammal. In front of this is the vesicle 

 (c) of the Third Ventricle (or Deutencephalon), which contains also the Tha- 

 lami Optici ; as development proceeds, this, like the preceding, is covered by 

 the enlarged hemispheres ; whilst its roof becomes cleft anteriorly on the 

 median line, so as to communicate with the cavities which they include. Still 

 more anteriorly (6) is the double vesicle (or Prosencephalon) which represents 

 the hemispheres of the Cerebrum ; this has a cavity on either side, the floor 

 of which is formed by the Corpora Striata, and which has at first no opening 

 except into the third ventricle ; the "fissure of Sylvius" (which enables the 

 membranes of the brain to be reflected into the lateral ventricles) being formed 

 at a later period. In the small proportion which the Cerebral Hemispheres 

 bear to the other parts, the absence of convolutions, the deficiency of commis- 

 sures, and the general simplicity of structure of the whole, there is a certain 

 correspondence between the brain of the 

 Human embryo at this period, and that of 

 a Fish ; but the resemblance is much 

 stronger between the foetal brain of the 

 Fish and that of the Mammal ; indeed, at 

 this early period of their formation, the 

 two could scarcely be distinguished; and it 

 is the large amount of change which the 

 latter undergoes, as compared with the 

 former, that causes the wide dissimilarity 

 of their adult forms. At about the 12th & 

 week we find the Cerebral Hemispheres 

 much increased in size, and arching back 

 over the Thalami and Corpora Quadrigemina 

 (Fig. 276): still, however, they are desti- ** 



> , & , i ' i 3 c j.i Human Embryo of sixth week, enlarged 



tute of convolutions, and imperfectly con- about three times . fl} vegicle of corpora quad . 



nected by Commissures; and there is a large rigemina; &, vesicle of cerebral hemispheres; 

 Cavity Still existing in the Corpora Quad- c, vesicle of third ventricle ; d, vesicle for cere- 

 rigemina, which freely Communicates with bellum and medulla oblongata; e, auditory 



the Third Ventricle. In all these particu- vesicle; . f> olfactor y fossa ; h > liver ; * * c <*ai 

 lars there is a strong analogy between the 



condition of the brain of the Human embryo at this period, and that of the 

 Bird. Up to the end of the 3d month, the Cerebral Hemispheres present 

 only the rudiments of anterior lobes, and do not pass beyond that grade of 

 development which is permanently characteristic of the Marsupial Mammalia, 

 the Thalami being still but incompletely covered in by them. During the 

 4th and part of the 5th months, however, the middle lobes are developed from 

 their posterior aspect, and cover the Corpora Quadrigemina ; and the posterior 

 lobes, of which there was no previous rudiment, subsequently begin to sprout 

 from the back of the middle lobes, remaining separated from them, however, by 

 a distinct furrow, even in the brain of the mature foetus, and sometimes in that 

 of older persons. In these and other particulars, there is a very close corre- 

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