288 



EMBRYOLOGY. 



make their appearance. In view of these phenomena, and guided by 

 other comparative-anatomical considerations, many investigators 

 have advanced the hypothesis that in the case of the ancestors of 

 Vertebrates the fore gut has been pierced by a greater number 

 of clefts than is now to be observed even in the Selachians, and 

 further that degraded or metamorphosed remnants of them are still 

 to be found in the head- and neck-regions. 



VAN BEMMELEN has observed in embryos of various Sharks and Skates out- 

 pocketings of the lateral wall of the throat behind the last visceral arch, and 

 has interpreted them as rudimentary visceral clefts, which no longer succeed 

 in breaking through (fig. 155 nsd^). Subsequently there are developed out of 



them, by growth of the epithelium, glan- 

 dular organs, the supra-pericardial bodies 

 (BEMMELEN), which are similar in their 

 structure to the thyroid gland. Also in 

 the head-region, which lies in front of 

 the first visceral arch, a reduction and a 

 metamorphosis of clefts has, according 

 to the opinion of various observers, taken 

 place. DOHRN especially has propounded 

 several hypotheses of this kind, for which, 

 however, I do not find valid grounds : (1) 

 that the mouth has arisen by the fusion 

 of a pair of visceral clefts, (2) that the 

 olfactory organs are to be referred to the 

 metamorphosis of another pair of clefts, 

 a view which is also shared by M. MAR- 

 SHALL and several others, (3) that a dis- 

 appearance of gill-clefts in the region of 

 the sockets of the eye is to be assumed, 

 and that the eye-muscles are to be inter- 

 preted as remnants of gill-muscles. 



nsd 



Fig. 155. Diagram of the development of 

 the thymus, the thyroid gland, and 

 the accessory thyroid glands, and 

 their relations to the visceral pockets 

 in an embryo Shark, after DE MEUROK. 



sch', cA, First and sixth visceral pockets ; 

 th, fundament of the thymus ; sd, 

 thyroid gland ; nad, accessory thyroid 

 gland. 



In the Chick the visceral furrows 

 become visible in the course of the 



third day of incubation, only three pairs at first, but, at the end of 

 the sam day, a fourth pair is added. 



In human embryos the visceral furrows are to be seen most dis- 

 tinctly (figs. 157, 154) when the embryo has attained a length of 

 three or four millimetres (His). Outer and inner furrows are in 

 this case deeply excavated and separated from each other by only a 

 thin epithelial closing plate; they diminish in length from before 

 backward. Of the visceral arches which separate them, the first is 

 the largest, the last the smallest ; seen in frontal section they form 

 two rows converging below, so that the oro-pharyngeal cavity tapers 

 funnel-like into the intestinal tube. 



