EMBRYO OF 6 MM. STUDIED IN SECTIONS. 



249 



Sp.c. 



A .is 



Cu. 



V.R. 



Scler. 



sion, ///, IV, 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 extend^ 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 fourth branchial arch. Between the 

 third external cleft and the second, //, is 

 a still greater protuberance on the outside 

 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, 7, 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 com- 

 pletely separated in this section from the 

 body. It shows the cavity of the fore-brain, 

 F.B, bounded by the ectoderm of the med- 

 ullary wall, and on one side also shows 

 the thickening of the epidermis, Olf, which 

 forms the olfactory plate or plakode, which 

 is to become the lining of the nasal pit. 



Transverse Section of the Lower End of 

 the Embryo (Fig. 183). Our third section 

 is very near, the end of the series. Owing 

 to the curvature of the posterior end of the 



body of younger embryos (compare Fig. FIG. 183. PIG, 6.0 MM. TRANSVERSE SERIES 9, 

 165; pig, 7.5 mm.), sections taken in the 

 plane which we call transverse strike the 

 lumbar region so as to give longitudinal sec- 

 tions of the spinal cord and primitive seg- 

 ments. Figure 183, therefore, shows the 



cavity of the spinal cord, Sp.c, cut for a very long distance. At the upper and 

 lower ends of the section, the dorsal side of the spinal cord is cut, and accord- 

 ingly we see at these levels sections of the ganglia, G, on either side of the 

 spinal cord. In the middle of our section the ventral portion of the spinal cord 

 is cut, and here, therefore, the ventral roots, V.R, of the nerves are displayed. 

 The somites are clearly marked by the external configuration of the embryo, the 



SECTION 519. 

 A. is, Intersegmental artery. Cu, Cutis plate. EC, 

 Ectoderm. G, Ganglion, muse, Muscle-plate. 

 Scler, Sclerqtome, auct. Sp.c', Spinal cord. 

 V.R, Ventral nerve-root. X 50 diams. 



