TRANSVERSE SECTIONS OF EMBRYO OF 12 MM. 191 



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 

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

 sinus (compare Fig. 96, C. S; pig of 10 mm.). In the section the cervical sinus 

 displays 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 figures 116 and 117. Our section passes through 

 the roots of the second cervical nerve, TV. cerv. 2, and shows both the dorsal gan- 

 glion and the ventral root arising from the ventral zone. These two roots join 

 and form the nerve-trunk, Nv. 2, which almost immediately divides, sending one 

 branch vertically upward into a mass of denser crowded cells (the anlage of the 

 dorsal musculature) and a ventral branch which descends almost vertically to- 

 ward the pharynx. Just inside of this ventral branch we have the section of the 

 vertebral artery, Art. v. Between the dorsal summit of the ganglion and the 

 spinal cord there is a minute bundle of nerve-fibers not shown in the figure. 

 These fibers constitute the commissural trunk of the eleventh nerve. The third 

 gill cleft is cut almost symmetrically, and extends from the 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 epithe- 

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

 intact in mammals. In the ichthyopsida the membrane becomes ruptured dur- 

 ing embryonic life, and the gill cleft is opened to the exterior. At the end of the 

 cleft the entoderm has undergone a special growth forming a distinct mass, TIivi, 

 on the side of the cleft toward the head. This entodermal structure is the anlage 

 of the thymus gland and is already penetrated by small blood-vessels which are 

 perhaps not capillaries, but sinusoids. The student should clearly understand 

 that the median region of the third gill cleft is really the pharynx proper. From 

 its median ventral line arises the beginning of the trachea, Tr, which should, per- 

 haps, be already designated as the anlage of the larynx. The entoderm extends 

 down in the median line for a considerable distance, making a figure which, in 

 the section, is shaped somewhat like a spear-head. In the center of the section 

 appears a small cavity. Further down toward the lungs we have only an epi- 

 thelial plate with no cavity observable in it (Fig. 119, Tra), the entoderm 

 of the trachea at this stage forming a solid cord. Ventrad from the trachea, 

 in the median region and between the two aortic arches, is a small, irregular, 

 deeply stained mass of cells, Thyr, the anlage of the thyroid gland. These 

 cells are entodermal, the anlage having been developed by a downgrowth of the 

 epithelium of the floor of the pharynx, although at the present stage the 



