SECRETING STRUCTURES. 33 



which effects this. The secretion contained in the cavity of the 

 cell appears to be the product of the solution of successive deve- 

 lopements of the nucleus, which hi some instances contains in its 

 component vesicles the peculiar secretion, as in the bile cells of 

 certain mollusca, and in others becomes developed into the secre- 

 tion itself, as in seminal cells-. In every instance, the nucleus is 

 directed towards the source of nutritive matter, the cell wall is 

 opposed to the cavity into which the secretion is cast. This ac- 

 cords with that most important observation of Dr. Martin Barry? 

 on the function of the nucleus in cellular developement. 



I have also had an opportunity of verifying, and to an extent 

 which I did not at the time fully anticipate, the remarkable vital 

 properties of the third order of secretions, referred to in the me- 

 moir to which I have just alluded. The distinctive character of 

 secretions of the third order is, that when thrown into the cavity 

 of the gland, they consist of entire cells, instead of being the 

 result of the partial or entire dissolution of the secreting cells. 

 It is the most remarkable peculiarity of this order of secretions 

 that, after the secreting cells have been separated from the gland, 

 and cast into the duct or cavity, and therefore no longer a com- 

 ponent part of the organism, they retain so much individuality 

 of life, as to proceed in their developement to a greater or less 

 extent in their course along the canal or duct, before they arrive 

 at their full extent of elimination. 



The most remarkable instance of this peculiarity of secretions of 

 this order, is that discovered by my brother, and recorded by him 

 in a succeeding chapter.* He has observed that the seminal se- 

 cretion of the decapodous crustaceans undergoes successive deve- 

 lopements in its progress down the duct of the testis, but that it 

 only becomes developed into spermatozoa after coitus, and in the 

 spermatheca of the female. He has also ascertained, that appa- 

 rently for the nourishment of the component cells of a secretion 

 of this kind, a quantity of albuminous matter floats among them, 

 by absorbing which they derive materials for developement after 

 separation from the walls of the gland. 



This albuminous matter he compares to the substance which, 



* See Page 39. 



