EMBRYO OF p MM. 217 



in the series of sections, it may be found that the condensed mesenchyma sur- 

 rounds the notochord, which, therefore, passes through the central portion of 

 each vertebral anlage. The vertebrae at this stage are entirely without any 

 distinct limitation and merge into the surrounding loose mesenchyma. Near 

 the anterior border of each nerve-trunk, and usually somewhat toward the me- 

 dian side of it, lie the intersegmental vessels, which are of small size and vary 

 greatly in their exact position and number, according as they are more or less 

 branched. Between the ends of the vertebral bows outside of the nerve-trunks 

 can be seen with higher power clusters of elongated cells with developing muscle- 

 fibers which are here still segmentally arranged between the processes of the 

 developing vertebrae. 



Pig Embryo of 9 mm. 



Pig embryos of this stage supplement very instructively those of 1 2 mm. It 

 will, of course, be advantageous for the student to prepare serial sections himself. 

 When that is not possible, there should at least be sections prepared for the lab- 

 oratory which the student may examine. Five sections are illustrated and 

 described below. They have been chosen to supplement the descriptions of the 

 sections of the pig of 12 mm., and they will be found to illustrate certain funda- 

 mental morphological relations in the embryo more clearly than older stages. 



Transverse Section through the Region of the Branchial Arches. — The bran- 

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

 separated from one another by deep ectodermal depressions, figure 129, 7, //, 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. 129) 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, A^. Just 

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

 the accessory nerve. The secondary segment, My, is sharply defined and has a 

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

 of the secondary segment is stained more lightly than the neighboring tissue 

 corresponding to the modifications which the cells are undergoing preparatory 

 to their change into young muscle-fibers. In the 12 mm. pig in this region the 

 cells of the muscle plate have already broken apart and no distinct plate can any 

 longer be recognized. Below the muscle plate follows the section of the jugular 

 vein. Jug. Lower down and in the median line we have the section of the 

 pharynx, Ph, lined by the epithelial entoderm. The pharynx is surrounded by 



