1877.] and the conducting parts of an acinous gland appear. 81 



chiefly been made use of to frame a theory of their mode of origin. 

 Notwithstanding the secondary and purely mechanical office that 

 the ducts of a gland perform in the process of secretion, it is to 

 those parts of the gland that, in the embryonic development, the 

 largest germinal powers have been assigned. The exaggerated im- 

 portance that has been attributed to the ducts in the history of a 

 gland's development depends, no doubt, on the fact that the ducts 

 of an acinous gland are the first parts of it that make their 

 ap2:)earance. In the embryo the first indication of the future gland 

 is a number of ducts extending, in a branching system, from a 

 point on the surface into the tissue beneath. In the generally 

 received explanation of the growth of the gland, the whole secret- 

 ing structure is taken as growing out, by a process of budding, 

 from the blind ends of the branching ducts. A germinal property 

 is attributed to such linear tracts of cells; their terminal expan- 

 sions are to be endowed \yith the property of secreting, although 

 they themselves will have the merely mechanical property of 

 conducting. In accordance with this view of the embryonic 

 development the general plan of structure of a gland is taken to 

 be that of a complex reduplication of the plane surface — skin or 

 mucous membrane — upon which the secretion is discharged. 



But the early appearance of the ducts in the embryonic 

 development, which has thus determined the theory of gland- 

 formation, is a reversal of the order in which the parts of a gland 

 respectively appear in the evolution of the more complex glandKS 

 from the simpler forms. The simplest glands may be said to be 

 masses of secreting cells, with a merely rudimentary and casual 

 provision for discharging the secretion; and the progress of the 

 gland towards a higher type consists in the acquisition of a more 

 permanent and more convenient system of conduits, or in a more 

 rigid division between its secreting and its conducting parts. It is 

 the object of this communication to show that the plan of structure 

 of a complex gland is best explained by tracing the gland back, 

 not to its embryonic form in the individual, but to its primitive 

 condition in the series of animals; and further, to show that the 

 inverted order in which its parts respectively occur in the embryo 

 of the higher animal is a fact capable of explanation according to 

 biological principles. 



There are several familiar examples of glands that are of a 

 complex type in the higher animals, but of a simpler arrangement 

 in the lower. The simpler type is, generally speaking, the 

 follicular. The salivary glands are follicular in insects, and 

 acinous in higher animals. The lachrymal glands are follicular in 

 Chelonians, but acinous in other vertebrates. J. Miiller, to whom 

 the last-mentioned fact is due, states also that he " has demon- 

 strated the transition of the pancreatic coeca of fishes through a 



