178 STUDY OF PIG EMBRYOS. 



primarily distributed, and, therefore, it is in a portion of this layer that we find 

 the ascending trigeminal tract situated. Next to the ectoglia comes the middle 

 layer, in which the neurones of the medullary wall are situated, and which is, 

 therefore, termed the neurone or gray layer (cinerea), easily recognizable under 

 the microscope by its brighter color, which is due principally to the fact that the 

 nuclei in this layer, though numerous, are much less crowded than in the inner- 

 most of the three layers, or primitive ependymal layer, which at this stage is 

 quite thick. Owing to the presence of nuclei, the gray layer is, of course, stained 

 much more than the ectoglia. The nuclei of the brain-wall show as yet very 

 little differentiation. There are numerous mitotic figures which are situated 

 exclusively close to the inner surface of the brain- wall in the fore-brain. The 

 structure of the fore-brain is similar, but the development is less advanced; the 

 differentiation of the neurone layer is only just beginning, and it has acquired 

 little thickness. In the hind-brain we see in the interior, along the region be- 

 tween the otocysts, a series of curved notches which impart a scalloped outline to 

 the wall. A distinct point separates one concavity from the next. Each one 

 of the spaces between two of the projecting points is designated as a neuromere. 

 The neuromeres correspond in number and position to the neighboring primitive 

 segments, and are, therefore, to be designated as segmental structures. They 

 also bear an evident relation to the development of the nerves, and the accepted 

 hypothesis is that from each neuromere springs a single nerve. The attempts 

 which have been made to verify this hypothesis have met with very serious diffi- 

 culties, for the relations are extremely complicated, and until the matter shall 

 have been much more thoroughly investigated than at present, we must remain 

 in the dark as to the precise morphological value of the neuromeres. But, inas- 

 much as they appear with the greatest constancy in the embryos of all verte- 

 brates, we cannot help accepting the view that they are really structures of fun- 

 damental importance. At the stage we are studying the neuromeres have already 

 begun to lose their distinctness, and in slightly older pigs can be traced only with 

 difficulty. In younger stages their primitive characteristics are better shown 

 (compare page 229). As regards the blood-vessels in the present section : there 

 are small branches of the veins, which show outside of the ganglionic commis- 

 sure, com; parts of the jugular vein appear in close proximity to the trigeminal 

 ganglion, and again at the side of the head. In the median line between the fore- 

 brain and hind-brain, or nearer to the layer, appears a section of the basilar 

 artery. Near the fore-brain on either side is the loop of the carotid artery. There 

 are several important points to be observed in the region between the trigeminal 

 ganglia and the fore-brain. In order to show these more clearly, a separate illus- 

 tration (Fig. 1 14), on a larger scale, of this portion of the section is given. The 



