184 STUDY OF PIG EMBRYOS. 



Section through the First Gill Cleft and Optic Evaginations . The section 

 shows on the dorsal side the upper cervical region of the spinal cord, on the ven- 

 tral side the fore-brain giving off the optic nerves. In this and the three sections 

 next following the complicated pharynx appears in various forms. The general 

 shape of the pharynx has been described with the aid of a figure of a wax model 

 of the pharynx made from the same series of sections from which these figures are 

 taken. The shape of the pharynx and of its four pairs of lateral pouches at this 

 stage is remarkably constant, so the student is not likely to encounter any serious 

 difficulty in identifying the parts. The spinal cord is oval in the section. Its 

 cavity has expanded in the middle. The lateral walls are quite thick, the median 

 ventral wall is thinner, and the median dorsal wall (deck-plate) is very thin. 

 The three primitive layers of the medullary tube are very clearly marked out, tfie 

 ectoglia appearing light, the ependymal layer appearing dark. The differentia- 

 tion is much more advanced on the ventral side of the spinal cord than on the 

 dorsal side, and, indeed, it is only in the ventral part that the three layers are 

 perfectly differentiated. In the median ventral line we have the floor-plate, in 

 which we can distinguish only two zones, while in the deck-plate there is no 

 differentiation of layers whatever. The spinal cord is clearly divided into a 

 dorsal zone, D. Z, and a ventral zone, V. Z, on each side. The two dorsal zones 

 are connected across the median line by the thin deck-plate, and the ventral 

 zones similarly by the thin floor-plate. The lower or ventral limit of the dorsal 

 zone is marked by the entrance of the dorsal or ganglionic root and by the fibers, 

 which represent the outgoing lateral roots. In the actual section figured, the 

 lateral roots, L. R. //, are those which enter into the formation of the eleventh 

 nerve. The true dorsal root does not appear in the figure . Internally the divi- 

 sion between the two zones is marked by the lateral angle of the central cavity 

 shown in the section. In the dorsal zone the differentiation of the three layers 

 has made slight progress. In the ventral zone, however, the development is far 

 more advanced. The most characteristic feature of this movement is the growth 

 of the cinerea or neurone layer, which increases in a twofold manner: first, by 

 encroaching upon the inner or ependymal layer; and, secondly, by the growth 

 of its constituent elements. Examination with a high power shows at once that 

 the cells have grown very much. Their nuclei are larger, granular in appear- 

 ance, rarely with any indication of a distinct nucleolus. Most of the cells are 

 neuroblasts and have well-marked protoplasmic bodies, finely granular in tex-' 

 ture. They have many of them already produced long, slender outgrowths 

 which we can identify as the neuraxons. In order to study the distribution of 

 the neuraxons and the form of the neuroblasts, it is necessary to apply the Golgi 

 rapid method, by which it can be demonstrated that a portion of the neuraxons 



