EMBRYO OF g MM. 219 



gill cleft, cl. Ill, a narrow, slit-like cavity lined by entoderm. On the left-hand 

 side of the embryo the junction of the entoderm of the internal pouch with the 

 ectoderm is shown. The two germ-layers have united to form a typical closing 

 plate. Above the third gill cleft the outline of the embryo shows a deep depres- 

 sion which is due to the commencing formation of the cervical sinus. From the 

 upper end of this depression runs upward the ectodermal fourth cleft, and from 

 its lower part extends downward the ectodermal third cleft. Between the third 

 and fourth clefts the external surface of the embryo protrudes somewhat. This 

 protuberance corresponds to the so-called third branchial arch. Between the 

 third external cleft and the second, II, is a still greater protuberance on the out- 

 side of the embryo. This marks the third branchial arch. The third aortic 

 arches are somewhat imperfectly shown, but the connection of the left third 

 arch with the central aorta appears. Between the second and first external 

 clefts we have the second or hyoid branchial arch, Hy; and, similarly, between 

 the first or auditory cleft, /, and the oral fissure, which separates the head from 

 the body of the embryo, we have the very large and protuberant mandibular 

 arch, Mdb. The head of the embryo is completely separated in this section 

 from the body. It shows the cavity of the fore-brain, F. B, bounded by the ecto- 

 derm of the medullary wall, and on one side also shows the thickening of the epi- 

 dermis, 01} , which forms the olfactory plate or plakode, which is to become the 

 lining of the nasal pit. 



Sagittal Section to the Right of the Median Plane. — In the accompanying 

 figure 130 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, 

 are 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 cavities do not 

 show, the position of these vessels is marked by the darker line of tissue. The 

 origin of one of these intersegmental vessels from the dorsal aorta, Ao, is indi- 

 cated in the lower part of the figure. The Wolffian body, W. b' , W. b", extends 

 from the level of the lungs and liver well down toward the pelvic end of the em- 

 bryo. Its ventral limit is marked by the body-cavity, Cos, and it is, of course, 

 covered by a layer of mesothelium, msth, which here, as everywhere and at all 

 stages, forms the boundary of the ccelom. In the Wolffian body we distinguish 

 readily numerous sections of the epithelial Wolffian tubules, and toward the 

 ventral side of the organ the characteristic glomeruli, Glo. Between the glom- 



