FROM TWELVE TO THIRTY-SIX SOMITES 



151 



primary fore-brain; but, owing to the simultaneous lateral expan- 

 sion of the telencephalon, the line of separation in the lateral 

 walls forms a curve with the convexity directed posteriorly 

 (Figs. 83 E and 86). 



(c) The next stage in the differentiation of the telencephalon 

 (20 s to 36 s) is characterized by a rapid expansion and evagina- 

 tion of its lateral w^alls, while the entire median strip extending 

 from the velum transversum to the recessus opticus remains prac- 



FiG. 85. — Optical sagittal section of the head of an embryo of 22-23 s. 

 The heart is represented entire. 

 Atr., Atrium. Hyp., Hypophysis. Inf., Infundibiilum. Md., Man- 

 dibular arch. or. pl.^ Oral plate. Pr'o. G., Preoral gut. Th., First in- 

 dication of thyroid. T. p., Tuberculum posterius. V. tr., Velum 

 transversum. Other abbreviations as before 



tically unaltered; and thus acts like a rigid band stretched over 

 the surface between these tw^o points. The effect of this is to 

 form a pair of outgrowths that soon begin to project dorsally, 

 anteriorly, and posteriorly (Fig. 83 E); these are the primordia 

 of the cerebral hemispheres, the cavities of which thus appear 

 as lateral diverticula of the median cavity of the telencephalon 

 (Fig. 86). The central part of the telencephalon may be called 

 the telencephalon medium, and the lateral outgrow^ths the hemi- 

 spheres. The walls of the hemispheres become considerably 

 thicker in this period, but quite uniformly at first, so that the 

 distinction between mantle and basal ganglia is indicated only 

 by position. (See Chap. VIII.) 



