Peripheral JSTervous System in Human Embryo 



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and the alar plates become wider, and at the same time flare apart. 

 As the alar plates spread apart the narrow dorsal seam, that exists be- 

 tween the two in the spinal region, widens out into the broad roof of the 

 fourth ventricle. The most striking feature of the rhombencephalon 



Fig. 1. Composite sagittal view of a 10 mm. human embryo (Huber col- 

 lection No. 3), showing the rhombencephalon and the so-called rhombic 

 grooves, which exist temporarily as transverse furrows in the floor of the 

 ventricle. They are connected by the fifth, seventh, ninth and tenth nerves 

 with the branchial arches. The fourth groove (d.) is an exception and has 

 no corresponding visceral nerve. Arising from it can be seen the sixth nerve ^ 

 extending forward to reach the anlage of the external rectus muscle. 



in this embryo is its large size as compared with the rest of the brain. 

 Its general form is shown in tbe drawing. It will be noticed that that 

 part of the alar plate cephalad to the trigeminal nerve, which is to take 

 part in the formation of the cerebellum, shows as yet little signs of dif- 

 ferentiation. 



