246 STUDY OF PIG EMBRYOS. 



Figure 178 shows the disposition of the cephalic and upper cervical nerves 

 and also the position of the nasal cavity, ' the eye, and the otocyst. 



Figure 180 gives an outline of the head and combines an indication of the 

 external modeling of the gill-arches, with a representation of the shape of the 

 pharynx. 



Pig Embryo of 6 mm. Studied in Sections. 



Of this stage three transverse sections are figured in order to give more exact 

 notions as to the structure of neuromeres, of the pharynx, and of the secondary 

 segments. 



Transverse Section through the Fourth Ventricle (Fig. 181). The section is taken 

 through the level of the head, and may be directly compared with figure 189. 

 The relations are so closely similar that it is unnecessary to describe the present 

 section (Fig. 181) in detail. The explanation of the figure is sufficient for the 

 identification of the parts. The otocyst is large and conspicuous, and the arrange- 

 ment of the nerves is essentially similar to what we find in the older embryos. 

 The neuromeres, however, are very distinct, especially those upon the left side 

 of the embryo, N. i, 2, 3, 4. Of these, the third is perhaps the most characteris- 

 tic. Each neuromere is separated from its fellow by an internal sharp ridge, so 

 that the inner boundary of each neuromere toward the cavity of the fourth ventricle 

 is a small arc of a circle. The cells are elongated and are placed radially to the 

 inner curved surface of the neuromere. A thin but distinct layer of ectoglia is 

 present. The light line, which marks the boundary between the adjacent neuromeres, 

 is produced by the comparative absence ol nuclei. As to the number of neuro- 

 meres our knowledge is still defective; nor have we yet succeeded in making sure 

 of their exact relation to the nerves of the head, though such a relation evidently 

 exists. Thus we find, for example, that the facial nerve is always connected with 

 neuromere 2 of our figure, and the glosso-pharyngeal nerve with neuromere 4. 



Transverse Section through the Region of the Branchial Arches (Fig. 182). The 

 branchial arches are much more conspicuous at this stage than in later ones, being 

 separated from one another by deep ectodermal depressions, figure 29, I, II, III, 

 IV; and, although /// and IV are already being turned in, preparatory to the 

 formation of the cervical sinus, they are still distinct and their order in the series 

 is evident. The section (Fig. 182) shows on the dorsal side the spinal cord, in 

 which we can already recognize the subdivision into dorsal zone, D.Z, and ven- 

 tral zone, V. Z. To the dorsal zone is appended the dorsal root; from the middle 

 of the ventral zone comes off the ventral root of a cervical nerve, N. Just between 

 the dorsal root and the wall of the spinal cord can be seen the section of the ac- 

 cessory nerve. The secondary somite, My, is sharply defined and has a distinct 

 growing edge showing at its upper limit in the section. The inner leaf of the 

 secondary somite is stained more lightly than the neighboring tissue, corresponding 

 to the modifications which the cells are undergoing preparatory to their change 



