250 ' STUDY OF PIG EMBRYOS. 



ectoderm, EC, forming an arch over the outside of each segment. Each mesodermic 

 somite shows three distinct parts: next to the ectoderm the broad, epithelioid 

 cutis plate, within which comes the spindle-shaped section of the inner portion of 

 the somite, muse, the anlage of the skeletal muscles; and, third, an expanding 

 mass of mesenchyma, Scler, which is sometimes termed the sclerotome. This term, 

 however, is not wholly felicitous, because this mesenchyma forms not only the seg- 

 ments of the skeleton, but the connective tissue of the whole region about the spi- 

 nal cord in the dorsal part of the embryo. . The figure shows very clearly that the 

 ganglia and ventral nerve-roots are arranged in exact conformity to the segments, 

 and it can be easily observed, by following through the series of sections, that for 

 each somite there is one ganglion and one ventral root. It also shows that the 

 ventral roots reach directly to the muscle-plate. The muscle-plate is histologically 

 partly differentiated, for its cells have already elongated in a direction parallel 

 with the longitudinal axis of the embryo, and their nuclei also have become much 

 larger than any other nuclei in the neighboring parts of the embryo, being per- 

 haps three times as large as the mesenchymal nuclei of the sclerotome. They are 

 oval in form, contain many fine, and usually one or two somewhat larger granules, 

 the larger ones being deeply stained; but the nuclei, as a whole, are stained more 

 lightly than their neighbors. Each somite is very clearly separated from its 

 neighbors, and between the ends of the adjacent muscle-plates there is a small 

 clear space entirely free from cells and extending outward to the epidermis. Just 

 inside of this space in every case is a small blood-vessel, the intersegmental artery, 

 A. is. The intersegmental arteries are small branches which arise in symmetrical 

 pairs from the dorsal aorta. 



Pig Embryo of 9 mm. Studied in Sections. 



Pig embryos of this stage supplement very instructively those of 12 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. Four 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 fundamental mor- 

 phological relations in the embryo more clearly than older stages. 



Sagittal Section to the Right of the Median Plane (Fig. 184). In the accorri- 

 panying figure 184 the cephalic end of the embryo is omitted; a portion of the 

 heart, the entire length of the Wolffian body, and the tail are included. The 

 dorsal outline of the embryo forms a characteristic curve. A long series of spinal 

 ganglia, G, is shown arranged in regular succession and following the curvature 

 of the back. The ganglia are easily recognizable by their dark staining; each of 

 them is so large as to occupy at least four fifths of the length of the segment to 

 which it belongs. The boundaries between the adjacent primitive segments are 

 indicated by the positions of the intersegmental arteries, A. is'. Even when their 



