TRANSVERSE SECTIONS OF EMBRYO OF 12 MM. 27 5 



olfactory chamber, Olf. The fore-brain has expanded laterally, L.V, to form the 

 lateral ventricles, the walls of which, H, are the anlages of the cerebral hemispheres. 

 On the dorsal side, which is the lower side in the figure, the hemispheres project 

 somewhat, leaving a median space between them. This median space is filled 

 with mesenchyma, which may already be regarded as the anlage of the falx. In 

 the tissue of the falx are two very small blood-vessels, the forward prolongations 

 of the lateral jugulars, which are to unite to form the median superior longitudinal 

 sinus. In the previous section these vessels also reappear, but are already united 

 (Fig. 192). In the median dorsal line the wall of the fore-brain is thin and shows 

 a characteristic notch. Close to the surface of the fore-brain there is a very dis- 

 tinctly marked vascular layer, the commencing pia mater, P.M, and with a high 

 power it can be easily seen that the differentiation of the arachnoid zone has 

 already begun. 



Section through the Third Gill-Cleft and Nasal Pits (Fig. 194). In this section 

 the head is clearly separated by a considerable space from the rest of the section. 

 The transverse diameter of the embryo is here much less than higher or lower, 

 so that the section as ay-whole seems somewhat narrow. It shows the entire 

 length of the third gffl^cleft, cl.iii, exhibiting, on one hand, its connection with 

 the median pharynx, and, on the other hand, its 'dorsal extremity, where its ento- 

 derm joins the ectoderm. The external outline of the embryo makes a deep de- 

 pression outside the end of the third cleft. This depression is the cervical sinus 

 (compare Fig. 166, C.S; pig of 10 mm.). . In the section the cervical sinus dis- 

 plays a narrow downward prolongation. If followed through in the series of 

 sections, this prolongation, which is on the inside of the hyoid arch, Hy, will be 

 found to connect with the second cleft. The spinal cord, Sp.c, presents essen- 

 tially the same structure as in 116 and 117. Our section passes through 

 the roots of the second cervical 'frv.2, and shows both the dorsal gan- 

 glion and the ventral rooi I zone. These two roots join 

 and form the nerve-trunk, 'ivides, sending one 

 branch vertically upward into a m; : < cells (the aniage of the 

 dorsal musculature) and a ventral branch which descfcods a!rpo> 

 toward the pharynx. Just inside of this ventral branch ''^^H 

 vertebral artery, Art.v. Between the dorsal summit of the ganglio- 

 cord there is a minute bundle of nerve-fibers not shown in the fi^. 

 fibers constitute the commissural trunk of the eleventh nerve. The third gill- 

 cl.iii, is cut almost symmetrically, and extends from t-he median line to the edge 

 of the section. It is lined throughout by the entoderm, which at the end of the 

 cleft on each side has met and fused with the ectoderm to form the epithelial 

 membrane, the closing plate. The membrane apparently normally remains intact 

 in mammals. In the ichti: -mbrane bee 1 ! ; --'rxg em- 



ft is OP -he cleft 



'he id'.-rgonc a special .on 



