STRUCTURE: AND DKYKI.OI-MKNT OK LKITDOSTKUS. 747 



into which the lumen of the third ventricle is prolonged -for a 

 short distance. 



The brain has become to some extent separate from the 

 superjacent epiblast, but the exact mode in which this is effected 

 is not clear to us. In some sections it appears that the separation 

 takes place in such a way that the nervous keel is only covered 

 above by the epidermic layer of the epiblast, and that the 

 nervous layer, subsequently interposed between the two, grows 

 in from the two sides. Such a section is represented in Plate 35, 

 'fig. 24. Other sections again favour the view that in the isolation 

 of the nervous keel, a superficial layer of it remains attached to 

 the nervous layer of the epidermis at the two sides, and so, 

 from the first, forms a continuous layer between the nervous 

 keel and the epidermic layer of the epiblast (Plate 35, fig. 25). 

 In the absence of a better series of sections we do not feel able 

 to determine this point. The posterior part of the nervous keel 

 retains the characters of the previous stage. 



At the sides of the hind-brain very distinct commencements 

 of the auditory vesicles are apparent. They form shallow pits 

 (Plate 35, fig. 24, an.} of the thickened part of the nervous 

 layer adjoining the brain in this region, Each pit is covered 

 over by the epidermic layer above, which has no share in its 

 formation. 



In many parts of the lateral regions of the body the nervous 

 layer of the epidermis is more than one cell deep. 



The mesoblastic plates are now divided in the anterior part 

 of the trunk into a somatic and a splanchnic layer (Plate 35, fig. 

 25, so., sp.\ though no distinct cavity is as yet present between 

 these two layers. Their vertebral extremities are somewhat 

 wedge-shaped in section, the base of the wedge being placed 

 at the sides of the medullary keel. The wedge-shaped portions 

 are formed of a superficial layer of palisade-like cells and an 

 inner kernel of polygonal cells. The superficial layer on the 

 dorsal side is continuous with the somatic mesoblast, while the 

 remainder pertains to the splanchnic layer. 



The diameter of the notochord has diminished, and the cells 

 have assumed a flattened form, the protoplasm being confined to 

 an axial region. In consequence of this, the peripheral layer 

 appears clear in transverse sections. A delicate cuticular sheath 



482 



