( 332 ) 



Tlio (irs( stages of development are only knowii lo me from 

 Hubrecht's paper. In the vonngest emljryo llie segregation of the 

 two licmisplieres has only jnst commenced. The next yonngest embryo 

 shows the hemisi)hei-es ah-eady develoi)e(l, namely at the lower 

 posterior peripliery of the anlci-ior vesicle. The}' are se[)arated tVoni 

 this latter l>y a sulcus hemisphaericns which anteriorly forms a i)retty 

 deep and sharj) gi-oove hut occipitally terminates in a shallow groove. 

 The segmcjitation of the posterior brain (5 segments) is clearly shown 

 especially by the youngest (Mubryo. 



The following stages are vei-y similar lo tiiose of other mammalian 

 oi'dei-s. in frontal sections thi-ough the hind-braiji the uncommon depth 

 of the sulcus limilans in ihc distal pai-|s is especially striking. Froutally 

 it sooJi becomes suioolh. A sulcus iiiicruKMliiis (( ii-ocidtergj is indicated. 

 The iniK'i' and oulei' labial grooves ("Lippcufurclicu"") arc present. The 

 cerebellum cousists of two syinmeli-ical lamellae, one to the right and 

 one to the left, joined by a thin and uai-row medial pai't. < )n the 

 outer sui-face of each lamella a broad uiedial longitudinal grooxe in 

 the immediate proxiuiily of the median part and a nai'rower but 

 relatively deeper latei'al groo\e avv to be seen. Corresponding to 

 these two grooves we find on the \enti'icular surface of each lamella 

 two longitiulinal ridges and three grooves (an unpaired sulcus 

 medianus dorsaiis, a sulcus uu'diali^ dor>alis aud a sidcus latei-alis 

 dorsalis on each side). The r()(»f of the meseuceplialou is i-alher |ioiided 

 and edged like a keel. The pharyngeal part of the hypophysis shows 

 an almost compact appendix, extending backwards and downwards. 

 Also two latei-al coutiiiuatioii> in a backward an<l «low iiward dii-ection 

 of the \entricle of the mid-braiii deserxe notice. 'J'lie cliorioid fissure is 

 already develo|)ed and siiows some bulgings. The sidcus hemisphaericus 

 has also become much more marked occipitally. The sicdde-fold 

 ("Sichelfalte") forms a sliar|i but shallow urooxc and is eucdosed by 

 the bifurcating xcntriide of the fore-brain. The sulcus hemisphaericus 

 lies at the right and left in close [)roximity of it. 'I'lie Amnion fold 

 (Hippocampal fui-row ) is still entirely absent. .Vt a stage which for 

 the i-est has oidy little advanced, the shape of the fore-brain has 

 alrea(h- materially progressed in development. The si(dde-fold is a 

 deep groove. In its wall the Amnion fold is noticed to which corre- 

 sponds on the surtace of the venlri(de a distinct Amnion ritlge. In 

 an occipito-parietal direction the sickle-fold reaches as far as the 

 anterior limit of the mid-l»i-ain, basally it iiiially lermiuates smoothly 

 in the lamina lerndnalis. The sidcus Moiiroi is \-ery sharply marked. 

 On one side it terminates smoothly in the iieiglib(»iirhood of the 

 stalk-fold of the optic xesicle and on the other side in the neigh- 



