Department of Anatomy, Nippon medical School
The so-called. sinus cervicalis covered with operculum is in the Streeter's horizon XIV (7mm long embryo) and XV (6mm long) visible, in which both three placodal formations for geni culate, extracranial and nodosum ganglion are distinct. In the next horizon XVI seem to become these formations ends of the branchial ducts 2, 3, 4. The branchial duct 4 is already cut off from the body surface so that it remains as vesicula cervicalis 4 rudimentary in the position caudal to the 4th branchial artery in contact with the 4th pouch, which is beginning to produce parathyreoid 4 and thymus or lateral thyroid in our 9mm long embryo (horizon XVI). Ductus branchialis 2 and 3 are open to the body surface, being in contact with the 2nd and 3rd pouch respectively, the 2nd pouch is regressing, the 3rd producing parathyroid 3 and thymus. This stage of development, in which neck region of human embryo remarkable change undergoes, is to be especially marked for the formation of peripheral nerves, d.i phrenic to the septum transversum, median, ulnar and musculocutaneus for ventral, axillaris and radialis for dorsal portion of brachial plexus on the one hand, for the beginning of precartilageous change without muscle differentiation on the other.
In the next horizon XVII (12.6mm long embryo) there is no vesicule cervial 4 already, but the cervical versicle 2 is as rudimentary one in contact with the 2nd pouch and a little larger cervical vesicle 3 in contact with parathyroid 3 and thymus ventral, with ganglian noddosum dorsal. These two cervical vesicles in this horizon disappear completely in the next horizon XVIII (18 mm long embryo). These findings about neck surface do not agree with Garrett's findings, in which the second and fourth vesicle are observed.
The development of the auricle which starts in the subepitheliar mesnchym on the mandibular and hyoid branchial arches begins at the horizon XVI (9 mm long embryo). In earlier embryo such proliferation is. almost lacking. The so-called hillocks 1 to 6 seem to be not so distinct as Streeter points out, the mesenchymal condensations. are not always sharp outlined. In the beginning of mesenchym proliferation in 9 mm long embryo, there is no distinction of hillocks on the mandibular, but on the hyoid arch hillock 4 and 5+6 can be separated. This separation of hillocks on the hyoid arch disappears in the next horizon (12.6 mm), while on the mandibular there appears separation of 1, 2 and 3 hillock. The so-called fossa angularis of His becomes manifest in this embryo. In the next horizon XVIII (18 mm) these 6 hillocks become distinct and each of them begins to show characteristic form.
The formation of parathroid 3 and thymus from the third pouch, that of parathyroid 4 and lateral thyroid or thymus from the fourth pouch begins also in the horizon XVI (9 mm). The former third complex lies lateral to the thyroid gland, the latter complex 4 caudal a little distance from thyroid. In the next horizon XVII (12.6 mm) the complex 4 lies cranial to the isthmus of the thyroid so that it comes to the medial side of the thyroid lobe. The parathyroid 4 lies caudal, a littel distance to the parathyroid 3 in this stage, while in the next horizon XVIII (18 mm) the former comes almost in the same level as the latter.

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