178 



On the Rd'itionsliip existing [no. 4, nrw SERIES* 



plcmcntary for the nurture and development of a superior embryo. 

 It is unnecessary to compare the various structures that are met 

 with in the two kingdoms, as the results of the transformation of 

 cells. In both we find tubular tissues formed by the coalescence 

 of cells, sclerous tissues where the cells have been solidified by in- 

 ternal deposit, and in Ascidian Mollusca a considerable quantity of 

 cellulose, the substance proper of plants. Indeed one observer re- 

 lates, that cellulose can be detected by chemical reagents in the 

 corpora amylacea of the human brain ; and if we can therefore 

 credit Virchow, the appellation " timber headed" will after all have 

 a foundation in truth. 



We now come to consider seme of the functions of organic life, 

 premising that in all organs where these vital operations are car- 

 ried on, we find a true cellular structure retained, beginning with 

 absorption and assimilation, we find these processes varying, ac- 

 cording to the rank of the animal or plant, in the respective 

 kingdoms, but still agreeing in the constant cellular agency. 

 Amongst individuals of humble status, the nutritious matter is at 

 once received through the cell wall by the process of endesmose, to 

 be elaborated and applied in the cavity of the cell to the various 

 wants of the structure. This of all methods is the most simple, and 

 a key to the plans of absorption and assimilation in the higher di- 

 visions, inasmuch as any apparatus more intricate is merely super- 

 added for the production of a more elaborate nutritious fluid, and 

 its transfer to distant parts of the organism. In the Vertebrata 

 absorption and assimilation obtain in their most perfect state, as 

 we find in this group a distinct tubular system, solely concerned in 

 the manufacture and translation of the chyle or alimentary fluid. 

 Between this and the simple endesmose and assimilation of the ani- 

 malcule, there are many intervening gradations of process, and as 

 an example of an animal occupying an intermediate position be- 

 tween the two extremes, we shall briefly contemplate the internal 

 economy of an Annelide, the Hirudo medicinalis. In this creature, 

 nearly the whole of the visceral cavity is filled by the stomach, 

 which consists of an elongated tube with a number of lateral sacs. 

 Over these csecal appendages is spread a delicate net-work of the 

 terminal branches of the circulatory system, destined to receive by 



