. Anatomischer Anzeiger. Anatomy, Comparative; Anatomy, Comparative. 620 the zone of the ultiraobrauchial bodies, bring these structures in contact with the dorsomedial edge of the thyroid, and the fusion that results initiates a new epoch in the history of the caudal pharyngeal complex. The period at which the fusion takes place is doubtless somewhat variable, if the length of the embryo is taken as a criterion. In embryos 11 mm. (Figure 2), mm., mm., and mm the fusion had not yet occurred. In embryos 12 mm. (1), 12 mm. (2), mm., 15 mm. (1) and (2), and all larger embryos
. Anatomischer Anzeiger. Anatomy, Comparative; Anatomy, Comparative. 620 the zone of the ultiraobrauchial bodies, bring these structures in contact with the dorsomedial edge of the thyroid, and the fusion that results initiates a new epoch in the history of the caudal pharyngeal complex. The period at which the fusion takes place is doubtless somewhat variable, if the length of the embryo is taken as a criterion. In embryos 11 mm. (Figure 2), mm., mm., and mm the fusion had not yet occurred. In embryos 12 mm. (1), 12 mm. (2), mm., 15 mm. (1) and (2), and all larger embryos the fusion had occurred. The sequence may be given as fol- lows ; —12 mm. (1), 15 mm. (1), 12 mm. (2), mm., mm., 15 mm. (2), mm., 19 mm., etc. the series being arranged accor- ding to the fusion and amount of transforma- tion which the body has undergone. With the thyroid the ultimobran- chial (i. e., the body of the tripartite complex) fuses first upon its ex- ternal and caudal aspect. Subsequent growth of the thyroid surrounds it on all sides save on the dorso-raedial aspect where this derivative remains longest exposed. Before fusion occurs the caudal pocket or extension of the ultimobranchial complex is a hollow structure with relatively thick walls (figure 2) with an epithelium (syncytium?) not unlike that of the ventral pocket of the third pouch which becomes the thymus, as it appears at this stage and for some time subsequently. After the fusion, a transformation takes place, appearing at first upon the side where the fusion has occurred. The appearance is quite characteristic;. Fig. 5. Section througli the caudal pharyngeal complex, both sides, in Embryo No. ä, 19 mm., Gage collection. Photograph, x 40. C carotid artery; L lumen; P. IV. parathyroid IV ; Th. thyroid; Z7. ul- timobranchial Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these
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