V.] 



THE PANCREAS. 133 



the manner of their ultimate metamorphosis into the ordinary hepatic tissue, is 

 not as yet quite clear. If we suppose each solid cylinder to represent a duct 

 with its lumen almost, but not quite, completely obliterated, we should gain a 

 view agreeing very closely with that put forward by Hering on the structure 

 of the adult liver. 



During the fifth day, a special sac or pouch is developed 

 from the right primary diverticulum. This pouch, consisting 

 of an inner coat of hypoblast, and an outer of mesoblast, is 

 the rudiment of the gall-bladder. 



20. About the middle of the third day, the pancreas 

 (Fig. 42, p.) also appears, but its exact mode of origin is still 

 somewhat doubtful. 



According to G-otte (Beit. z. entwicU. des Darmcanals des Huhnchens) it 

 commences as a thickening of both the hypoblast and mesoblast of a portion of 

 the wall of the digestive canal on the same level as the left diverticulum of 

 the liver. The hypoblast in the centre of this thickening becomes hollow, 

 forming a cavity connected with the inside of the digestive canal by a narrow 

 opening. Around this cavity processes of hypoblast are seen on the fourth day 

 stretching into the surrounding mesoblast. These processes, which are at first 

 solid but afterwards become hollow and ultimately branched, are in the early 

 stages so completely covered up by mesoblast that they are not visible on the 

 exterior. The primary cavity elongates into the duct, the hollow processes 

 representing its branches* On the sixth day a new similar outgrowth takes 

 place between the primary one and the stomach. This, which ultimately 

 coalesces with its predecessor, gives rise to the second duct, and forms a 

 considerable part of the adult pancreas. A third duct is formed at a much 

 later period. According to this view, with which those put forward by Kolliker 

 and Remak in the main agree, the so-called 'secreting' cells of the pancreas 

 as well as the epithelial lining of the ducts are derived from hypoblast. 

 Schenk (Die Bauchspetiheldriise des Embryos. Anat. u. Physiol. Untersuch- 

 ung. Wien. S. I.) however is of opinion that the former originate in a 

 transformation of the mesoblast, the hypoblast giving rise to the epithelium 

 of the ducts only. 



Shortly after the first appearance of the pancreas, the 

 spleen appears as a thickening of the mesentery of the stomach 

 (mesogastricum) and is therefore entirely a mesoblastic struc- 

 ture. 



Its development has been recently investigated by Peremeschko (Sitz. der k. 

 Akad. in .Wien, Bd. 56, 1867) and by W. Miiller (Strieker's Histology). 



According to these investigators, the mass of mesoblast which forms the 

 spleen becomes early separated by a groove on the one side from the pancreas and 

 on the other from the mesentery. Some of its cells become elongated, and send 

 out processes which, uniting with like processes from other cells, form the trabe- 

 cular system. From the remainder of the tissue are derived the cells of the spleen 

 pulp, which frequently contain more than one nucleus. Especial accumulations 

 of these take place at a later period to form the so-called Malpighian corpuscles 

 of the spleen. 



21. The thyroid body is also formed towards the end of 

 the third day, in connection with the alimentary canal. 



