1877.] and the conducting parts of an acinous gland appear. 33 



different functional importance of the acini and the ducts respec- 

 tively. Ducts are always mechanical in purpose, and constant in 

 structure; but the secreting parenchyma of a gland is subject to 

 constant cellular changes, more or less obvious. In the periodical 

 changes of the mammary gland the constancy of the ducts as 

 contrasted with the variability of the acini is readily noticed. 

 According to a generalisation of Mr Herbert Spencer's [Principles of 

 Biology, Vol. i. 371) there are two modes of development, the direct 

 and the indirect. The indirect mode of development is for the 

 embryo what the cumbrous process of adaptation has been in the 

 progenitors ; and all embryonic development is therefore at the 

 outset indirect. But under certain circumstances the natural 

 indirectness of the development tends to be supplanted by a more 

 direct mode of formation. The embryo, or part of the embryo, 

 "grows to its appointed shape by the shortest route." Certain 

 classes of animals are distinguished by the directness of their 

 development, and certain organs and parts of the body are in like 

 manner distinguished. According to Mr Spencer, directness of 

 development comes in in the embryo, where in the progenitor 

 there has been constancy of conditions. If the two kinds of 

 structure in the breast are compared in this respect, the ducts 

 are those parts of the organ that are constant in their structure ; 

 if the explanation that has been given of the origin of ducts be 

 correct, their constancy and permanency is their raison d'etre. 

 But the acini of a gland, such as the mammary gland, are con- 

 stantly varying; their secreting activity depends for its existence 

 on incessant changes of their cellular elements. If, then, those 

 remarkable differences in the mature animal are reflected in the 

 embryonic development, the difference would be that the ducts 

 have a direct development, or, in Mr Spencer's words, that they 

 "grow to their appointed shape by the shortest route." It is 

 the directness of their development that has to be associated 

 with the fact of their early appearance ; but the explanation will 

 lose its force if the complex system of ducts is taken as growing 

 out from a central point by a process of budding. In the mamma 

 of the guinea-pig there is reason to think that the ducts are 

 laid down throughout its rudiment according to a pre-determined 

 plan, and that they are formed by linear aggregations of the 

 embryonic cells. The same mode of formation is referred to by 

 Goodsir when he says, of glands in general, that " in certain instances 

 it has been observed that the smaller branches of the duct are not 

 formed by continued protrusion of the original blind sac, but are 

 hollowed out independently in the substance of the blastema, and 

 subsequently communicate with the ducts" (Anatom. Memoirs, 

 Vol. II. 425). 



Limiting the attention to the mammary gland, of whose 



