EMBRYO OF 17 MM. 231 



comes the spindle-shaped section of the inner portion of the segment, 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 segments of the 

 skeleton, but the connective tissue of the whole region about the spinal 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 

 segment 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 segment is very clearly sepa- 

 rated 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 epi- 

 dermis. Just inside of this space in every case is a small blood-vessel, the inter- 

 segmental artery, A . is. The intersegmental arteries are small branches which 

 arise in symmetrical pairs from the dorsal aorta. 



Pig Embryo of 17 mm. 



Since the pig of 12 mm. contains the anlages of perhaps every important 

 part of the body, sufficiently advanced in development to be clearly recognized, 

 we find in the immediate subsequent development that we have to do not so 

 much with an introduction of new parts as with the differentiation of those which 

 have already commenced. Embryos of 17 mm. are convenient for the study of 

 the differentiations referred to. Particularly important for the student to note 

 are the advances in the development of the vertebrae, of the lungs, of the Wolffian 

 bodies and genital glands, and of the kidneys. These points are illustrated in 

 figures 136 to 138, representing portions of three transverse sections of a 17 mm. 

 embryo. 



Transverse Section through the Lungs. — The epidermis of the embryo has 

 become more distinct owing to its growth in thickness, which is accomplished by 

 the increase of the number of layers of cells. The growth is very marked at the 

 sides of the section about the level of the vertebra. At these points it can be 

 clearly seen that upon the outside the epidermis has a very thin layer of flattened 



