34 Prof. Maxwell, On a Paradox in the [Mar. 12, 



development in the guinea-pig I have published an account 

 {Journal of Anatomy and Physiology, Vol. xi.), the early appear- 

 ance of the ducts is not only no evidence that they are the 

 germinal tracts of the whole secreting structure, but it is, on the 

 other hand, a circumstance that might be expected when their 

 purely mechanical purpose and permanency of structure in the adult 

 is taken into account. Further, there has been no attempt on the 

 part of writers on development to demonstrate the actual outgrowth 

 of the acini from the blind ends of the ducts. The branching 

 system of ducts has been seen extending at an early period over 

 the area of the gland, and the unwarranted step has been taken of 

 assuming the still farther expansion or budding of the smallest 

 ducts to form the acini. In the paper above quoted I have 

 described the formation of the mammary acini as a process of 

 interstitial growth, whereby round spaces filled with secretory 

 cells are developed from the matrix tissue of the gland at many 

 scattered points, and at a time when the ducts are abeady there. 

 The developing acini therefore find themselves ranged along the 

 sides of ducts into which they burst, through the internal pressure 

 of their developing cells. The same conclusion as to the develop- 

 ment of the terminal secreting crypts of glands in general is stated 

 in a summary way by Goodsir (1. c). 



It thus appears that, in the only cases where the development 

 of the secreting structure has been specially looked for, it does not 

 support the theory of the germinal property of the ducts. That 

 remarkable endowment has been attributed to them no doubt on 

 account of their early appearance. In the case of the mammary 

 gland, the early appearance of the ducts has led to an erroneous 

 theory of its growth; and it becomes an interesting question 

 whether, in the case of other glands, the early appearance of the 

 duct, or ducts, implies, as has been always hitherto assumed, that 

 they are the germinal starting-points of the whole organ. 



March 12, 1877. 

 Peof. Clerk Maxwell, President, in the chair. 



The following communications were made to the Society : 



(1) Prof. J. Clerk Maxwell. On a Paradox in the Theory 

 of Attraction. 



Let A^^A^ be a straight line, P a point in the same, X^, X^ 

 corresponding points in the segments PA^, PA^. 



