(4) 



anew only at a much later stage Avhen the cortex has already become 

 amply lamellised. 



In the mean time, during the thickening of tiie lamina cerebellaris, 

 developmental phenomena haAe taken place in the bordering region 

 between Mesencephalon and Rhombencephalon, giving rise to the 

 formation of Plica encephali dorsalis (Kupfer), Isthmns rhombence- 

 phali and Velum mednllare anteriiis. In the youngest stage represented 

 (fig. 1) the anterior edge of the Lamina cerebellaris passes directly 

 into the mesencephalic roof, only the posterior edge of this latter 

 is a little inwardly in\'aginated. An Isthmns rhombencephali or Plica 

 cjicephali dorsalis do not yet exist. In the next stage of development 

 (fig. %() the mesencephalon has obtained a clearly defined posterior 

 Avail, vertical to the roof; the inward invagination of the posterior 

 edge of the roof is still in existence and is partly visible in fig. 3, 

 but has disappeared in . fig. 4 on account of tlie thickening of the 

 posterior wall of the mesencephalon. In fig. 2 the plica encephali 

 dorsalis has developed, bordered in front by the posterior wall of the 

 mesencephalon, at the back by the lamina cerebellaris. The formalioii 

 of the Plica is accompanied by a rotation of the Lamina cere- 

 bellaris, the anterior edge of which is now no longer situated at 

 the front but below and as a consequence of this the Isthmus 

 rhombencephali is now also indicated in principle. Next the bottom 

 of the plica encephali dorsalis becomes broader, there arises between 

 the thickened lamina cerebellaris and the mesencephalon a thin middle 

 plate (figs. 3a and 4), the first origin of the velum mednllare anterius. 

 The further details of this stage and the following stages will be 

 extensively described elsewhere. 



The lobulisation of the cerebellum in the second stage is characte- 

 rised by the fact that the grooves which divide the surface of the 

 cerebellum into several regions originate with a single exception in 

 the median plane and from there extend laterally. These interlobular 

 grooves are consequently unpaii'ed with one exception and divide 

 the foetal cerebellum of man into a number of lobes, which can be 

 homologised without difficulty with those which I learnt to be typical 

 for the adult mammalian cerebellum. 



The median section of a cerebellum with indications of the grooves 

 that appear fii-st, is given in fig. 5. The incisura fastigii has been 

 shifted more to the front compai-ed with fig. 4. On the extraventricnlar 

 plane two grooves can be clearly distinguished, one, a little rostrally 

 from the top of the cerebellum, another at a short distance from the 

 margo myelencephalicus. Which of these two arises first I have not 

 been able to make out, evidently they both arise about simultaneously. 



