EMBRYO OF 9 MM. 227 



ing is homologous with the ectodermal fold which produces the fin of fishes, or 

 at least that portion of the fin in which the fin-rays are developed. 



Frontal Section through the Second and Third Gill Clefts. In this pre- 

 paration (Fig. 133) the section hits the posterior wall, Ot, of the otocyst 

 and is just anterior to the origin of the glossopharyngeal nerve. The appear- 

 ance of the section of the hind-brain is characteristic for this region of young 

 embryos. The deck-plate has grown gradually in size and forms a wide mem- 

 brane, epen, the ependymal roof of the fourth ventricle. Owing to this growth 

 of the deck-plate, the upper or dorsal limits of the dorsal zones, D. Z, are brought 

 far apart and the cavity of the hind-brain is thus enlarged. The dorsal zone is 

 marked by an angle in the interior and by the point of entrance of the nerve- 

 roots on the exterior from the ventral zone, V. Z. On their dorsal side the dorsal 

 zones thin out and pass over gradually into the ependyma. The ependyma con- 

 sists of a single layer of cells. In the dorsal zone the differentiation of the three 

 primary layers of the medullary wall has scarcely begun, but in the ventral zones 

 the three layers are already distinguishable, though not far advanced in their 

 differentiation. In the floor-plate there are two layers. Below the medullary 

 tube lies the basilar artery, A. has, and below that, not far from the upper wall 

 of the pharynx, lies the small round notochord in the midst of loose mesenchymal 

 cells, which have not yet begun to condense themselves about the notochord. 

 The pharynx is a wide space of rather small dorso-ventral diameter, and having a 

 much thinner layer of entoderm on its dorsal than on its ventral side. Above 

 the pharynx on either side lies the section of the descending aorta, Ao. d. The 

 reference line to this vessel crosses a dark mass of cells which belong to the gan- 

 glion nodosum of the ninth nerve. Below the pharynx the section shows the 

 third aortic arch, Ao. j, and the fourth aortic arch, Ao. 4., just springing off from 

 the median aortic trunk above the heart, so that the two fourth arches are 

 connected across the median line. Between the third and fourth aortic arches 

 on either side is a small cavity lined by entoderm, cl. Ill, a diverticulum from 

 the third gill cleft. Immediately below the otocyst is the jugular vein, Jug. 

 From a point below the jugular there extends a prolongation, Hy, which may be 

 taken as a portion of the hyoid or second branchial arch. It extends downward 

 and consists of a mass of mesenchyma covered by ectoderm. It encloses a space, 

 cl. II. ex, which may be regarded as the external portion of the second gill cleft. 

 In a neighboring section (455) the prolongation of the pharynx shown in figure 

 133 can be traced still further until it opens into this space, cl. II. ex. The second 

 cleft is open upon both sides of the embryo, the first and third have closing mem- 

 branes, the fourth cleft is not yet so far developed that its entoderm has come in 

 contact with the epidermis of the embryo. The second cleft probably always 



