GLAND. 



493 



This development of ccecal 

 tubuli is seen in the liver of 

 Limmcus stagnate in tlie em- 

 bryo state, (fig. 223). In 

 the embryo of Lacerta vi- 

 ridis, (fig. 224,) the rudimen- 

 tary liver with its blind se- 

 creting canals (e) are observed; 

 the elongated heart (a) fur- 

 Jig. 224. 



Fig. 223. 



nishing the aorta (fe) dividing into its right and 

 left trunks, together with the principal venous 

 tnink (c), are represented ; d is the intestine, 

 J the rudiment of the corpora Wolffiana, and 

 g g the rudiments of the upper and lower 

 extremities. 



One of the most remarkable differences ob- 

 served in the development of the several glands 

 relates to the proportion between the mass of 

 the primary plastic substance, and the extent 

 and number of the contained tubes ; thus, in 

 the evolution of the liver there is seen a thick 

 layer of the primitive matter ; whilst, on the 

 contrary, the parotid gland in the embryo of a 

 calf two inches seven lines long, consists of a 

 tube visible to the naked eye, and not at all 

 covered by parenchyma. 



5. The mode in which the secondary tubes 

 are developed has been observed with great 

 care ; and it is distinctly established that they 

 do not proceed as mere elongations of the pri- 

 marif cavity, but are formed in an indepen- 

 dent manner. One of the latest writers on the 

 development of the body, Valentin* has 

 given a very exact account of the process in all 

 the glands. He states that in the neighbour- 

 hood of the chief duct or of a branch of it, 

 small oblong accumulations of the plastic 

 mass are formed, which become hollowed in 

 the interior, and these hollows, at first inde- 

 pendent of the principal cavity* subsequently 

 communicate with it. It is also observed by 

 Muller that in the kidney of Batrachian Am- 

 phibia, the secreting tubes first appear as 



* Loc. cit. p. 521, et alibi. 



vesicles which are formed before the ureter, 

 and therefore independently of the principal 

 duct.* As the tubes become more developed, 

 the plastic substance around them, by acquiring 

 greater firmness, constitutes their walls, and 

 thus determines their exact form and limits. 

 It is necessary to state that in every instance 

 without an exception, the newly-formed canals 

 end in ccecal extremities, which are often 

 rather swollen, presenting a pedunculated ap- 

 pearance. 



6. In proportion as the canals become 

 formed in the substance of the plastic mass, 

 this latter gradually diminishes in quantity, 

 till ultimately, when all the tubuli are formed, 

 it is so much reduced that it merely fills up the 

 interlobular fissures, and is in fact converted 

 into the interstitial cellular tissue. 



7. At the same period of time that the tubes 

 are thus being formed, the bloodvessels are 

 being developed ; and, as Muller and Valentin 

 remark, a very close parallel is presented in the 

 generation of these the essential parts of the 

 gland. As in the case of the tubes, there are 

 at first little masses, or islands, of the plastic 

 substance, which subsequently join together, 

 and their interior becoming liquified, a num- 

 ber of little channels are formed containing a 

 circulating fluid, and which channels, by the 

 subsequent consolidation of their walls, are at 

 length formed into perfect bloodvessels. Like 

 the tubes these vessels are at first independent ; 

 they afterwards open into larger trunks and 

 ultimately into the heart. It is proper to 

 remark that, although there is such a corres- 

 pondence in the process of development in 

 each instance, the bloodvessels are formed 

 quite independently of the canals ; that they 

 occupy a different part of the plastic mass; 

 and that they never present that continuity 

 which ought at this epoch to have been very 

 apparent, if the theory of Ruysch had been 

 founded in truth. 



8. The several glands are not developed 

 equally early, some having their organization 

 much more advanced than others; thus at the 

 time when the pancreas is so far formed as to 

 contain an immense number of canals, the 

 parotid presents only a single duct or a few 

 ramifications.f The principle which regulates 

 the relative degree of development has evi- 

 dently reference to the importance of the organ 

 during the foetal life ; and in this respect the 

 liver is most remarkable, for that body being, 

 as I conceive, the true decarbonising organ in 

 the animal kingdom, and therefore its func- 

 tions being doubtless necessary in the foetus, 

 very quickly acquires a high degree of organi- 

 zation, so much so that, as we learn from all 

 observations, it very speedily fills the greatest 

 part of the abdomen. J 



* De Gland. Struct, p. 87. 



t Rathke in Burdach s Phy. II. Band. p. 576. 

 Valentin, 1. c. p. 225. 



$ In the embryo of a sheep five lines in length, 

 Valentin has found the liver filling half of the 

 abdominal cavity ; and in the embryo eight lines 

 long, that organ constitutes three-fourths of the 

 bulk of the viscera contained in the peritoneum: 



