

TRANSVERSE SECTIONS OF EMBRYO OF 12 MM. 273 



connection with the body of the embryo. The space O.F, between the head piece 

 and body piece, may be designated as % the oral fissure, since it is into this space 

 that the mouth opens. In general there is considerable resemblance between this and 

 the section last described, but in the present section the eyes have disappeared 

 and we get the first indications of the nasal pits, Olf. That on the left side of 

 the body shows a trace of the cavity of the pit. The posterior part of the pharynx, 

 Ph, is cut in the section, instead t of the anterior part as in figure 192. The first 

 gill-cleft does not show, but the second cleft, cl.II, does. It lies posterior to the 

 first cleft and therefore appears higher up in the figure. The spinal cord, Sp.c, 

 shows the same general structure as in the previous section. On either side of it 

 may be seen the small and inconspicuous root of the eleventh or accessory nerve. 

 It could not be properly represented in the figure. Some distance below the 

 cord lies the small circular section of the notochord, which differs so slightly .in 

 staining from the surrounding mesenchyma that it cannot be well made out 

 without the use of a higher magnifying power. It is enclosed by a distinct mem- 

 brane which is thick enough to produce a double outline, and contains a consid- 

 erable number of scattered nuclei, which are, however, nowhere much crowded. 

 The nuclei are round in form, decidedly larger than those of the surrounding 

 mesenchyma, granular, and containing each several more conspicuous, darkly 

 staining granules. There is a very slight gathering of mesenchymal cells about 

 the notochord, as if to form the anlage of a sheath. Just below the noio< 

 there is a broad band of somewhat daTker staining, due to a greater conden? 

 of the mesenchyma in that region, and this condensation represents the beginning 

 of the formation of the vertebral structures. On either side we find the trans- 

 formed myotome, My, or anlage of the striated muscular tissue. This tissu 

 produced from the 'secondary somites of earlier stages. The cells have now sepa- 

 rated from one another, . have lost their distinctly segmentai grouping, and 

 begun to elongate into true muscle-fibers. All that can be distinguished 

 the low power is the somewhat darker appearance of this part of the section, 

 to the great crow'' at: of the nuclei. Between the muscular anlage and the noto- 

 chord ' shows a portion of the first cervical nerve, N.cerv.i, and just 



; nerve is a small blood-vessel not represented in the figure. Th 

 a similar blood-vessel symmetrically placed on the opposite side. They ar^ the 

 small vertebral arteries. The anterior cardinal veins, Card, are large and conkpieu- 

 ous vessels, but despite their size they have merely endothelial walls and thl; 

 no condensation of thb^ mesenchymal cells around them, although such a I 

 densation is to take place later to form the anlages of the muscular and char 

 ive-tissue coats (media and adventitia) of the adult. On the dorsal side o* : 'e 

 cardinal vein and close to it is a light spot in which can be easily distingu ' 

 \vith the high power, several more or i-= s distinct bundles of nerve-fiber? whk 

 separated from one another by mesenchyma 1 cells. For this reason it is 

 \vhat difficul 1 to recognize this ner\ viih ti T or to at it 



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