DEVELOPMENT OF THE ALIMENTARY CANAL. 107 



developed as a very thin layer which separates from the sides and bottom 

 of the primitive groove ; but contrary to Bischoff's observations, he states 

 that the formation of nerve-substance does not take place until the primi- 

 tive groove is converted into a canal by the junction of the margins of 

 the lateral masses of cell-substance by which it is bounded. 



Development of cerebral hemispheres. It is stated by Professor Ret- 

 zius* that the three portions of the cerebral hemispheres in the human 

 embryo are developed, not at once, but at three separate periods. In the 

 first of these periods, which extends from the second to the third month, the 

 anterior lobes are formed ; during the second period, which is comprised 

 between the end of the third and the beginning of the fifth month, the 

 middle lobes are formed ; after this, therefore last of all, the posterior 

 lobes are developed, 



The inferior horns of the lateral ventricles and the hippocampi do not 

 appear until the second period ; at this period also the optic thalami make 

 their appearance, and after these the tubercula quadrigemina. 



Development of the Alimentary Canal.\ 



A somewhat different account of the mode of development of the Ali- 

 mentary system of the toad to that given by ReichertJ of the same pro- 

 cess in the frog, has been furnished by Vogt. In the opinion of the 

 former observer, the Alimentary canal is formed from the central cells of 

 the yolk, but according to Vogt, it is formed from the internal or so called 

 mucous layer of cells of which the germinal membrane or cortical part 

 of the yolk is composed. On making a longitudinal section of the em- 

 bryo and yolk through the chorda dorsalis, at the time of the formation 

 of the branchial arches, the central part of the yolk (dotter-kern) is 

 observed as a loose globule surrounded by a tolerably thick layer of 

 cortical cell-substance. In the midst of this cortical layer the chorda 

 is imbedded, being separated from the nucleus (or central part) of the 

 yolk by a considerable quantity of the cells. In front, immediately beneath 

 the elongated cephalic portion of the axis, there is observed a slight 

 depression, which is the rudiment of the cavity of the mouth. At this 

 time no separation of the cortical layer of cells into serous and mucous 

 laminae has taken place. As the growth of the branchial arches deve- 

 loped from the undivided cortical layer proceeds, the portion of the yolk- 

 nucleus corresponding to the rudimentary mouth becomes depressed. 

 The division of the germinal membrane into its serous and mucous layers 

 now gradually commences. The latter separates itself from the former 

 over its whole extent, except at the part corresponding to the rudimentary 



* Arch. d'Anat. Gen. et de Phys. Janvier, 1846, p. 24. 



t Miiller's Physiology, page 1633. I Ibid. p. 1527. 



Entwickelungs-ges. der Geburtshelferkrote, p. 67. 



