204 STUDY OF PIG EMBRYOS. 



a neuromere. The neuromeres correspond in number and position to the neighbor- 

 ing 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 J 

 V accepted hypothesis is that from each neuromere springs a single nerve. The at- 

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

 difficulties, 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 vertebrates, 

 we cannot help accepting the view that they are really structures of fundamental 

 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 

 246). As regards the' blood-vessels in the present section: there are small branches 

 of the veins, which show outside of the ganglionic commissure, com; parts of the 

 cardinal 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 illustration (Fig. 190), on a larger 

 scnle, of this portion of the section is given. The trigeminal ganglion, the wall of 

 the foio-brain, and the wall of the hind-brain will be at once identified, so that the 

 correspondence with the general figure is easily followed. Between the trigeminal 

 ganglion and the fore-brain are four veins, two of . which, Card' and Card'" ', are 

 larger and are parts of the main cardinal stem passing from the region of the 

 hind-brain ^to thai- of the, fore-brain, while the ;-, u .. mailer ones, Card", are merely 

 nches of the same vessel. Close to the section, Card'", of the cardinal nearest 

 the fore-brain lie the very small sections of the fourth, N.4, and third, N.T,, cere- 

 bral nerves. The fourth nerve is minute in size and lies just behind the vein. 

 The third nerve, thougfi somewhat larger, is also very small and lies anterior to 

 the vein somewhat on its medial side. Both of these nerves, owing to their 

 small dimensions, are somewhat difficult to observe with the low power. The de- 

 tailed figure brings out more clear/ other points. It shows very clearly the junc- 

 tion of the trigeminal. A - , -tico-facial, N.j,8, ganglia with the wall of the 

 1 brain, and also the div^ 'of that wall into i' three primary layers, the 

 ' "/, the gray layer ..rid .iie inner or ependym layer, ^Epen, and 

 floor-plate ' :, /<7r/>/?. Immediately below it is the basilar 

 'n is the s< I loop of the carotid, 

 "nd oi thv basilar artery, which 

 <y symmetric" ' ' 

 spond 



