independerj 



SIMPLE ISOLATED CELLS. REPRODUCTIVE CELLS. 151 



independent animalcules ; it is now generally admitted, however, that 

 they have no more claim to a distinct animal character, than have the 

 ciliated epithelia of mucous membrane, which will likewise continue in 

 movement when separated from the body. Similar bodies are formed 

 by all the higher Cryptogamic Plants ; and it appears from late re- 

 searches that their office, as in Animals, is to fertilize the contents of 

 the " germ-cells," with which their self-moving power brings them into 

 contact (CHAP. XL) It is a curious fact that the seminal cells, in which 

 the Spermatozoa are formed, are ejected from the gland in certain Crus- 

 tacea, not only before they have burst and set free their Spermatozoa, 

 but even long before the development of the Spermatozoa in their inte- 

 rior is completed; thus affording a complete demonstration of their 

 independent vitality. 



242. The " germ-cells," in like manner, are very commonly developed 

 among the lower Animals as the epithelia of the tubes or follicles which 

 constitute the ovary ; but in Man and the higher Animals, the ovary is 

 a solid organ, and the germ-cells are developed in its substance, lying 

 in the midst of the dense fibrous tissue which forms its parenchyma. 

 These germ-cells, which are known as " ovisacs," like the sperm-cells, 

 develope secondary cells or ova in their interior ; each ovisac, however, 

 producing but a single ovum. The ovum, again, contains a tertiary cell, 

 the germinal vesicle, whose contents appear to mingle with those of the 

 sperm-cell in the act of fecundation, so that the fertilized germ is the 

 result ; the remaining contents of the ovum being the nutritive materials, 

 at the expense of which this germ undergoes its first development 

 (CHAP, xi.) 



243. We now proceed to a class of cells, which are equally indepen- 

 dent of each other, which begin and end their lives as cells, without 

 undergoing any transformation, but which form part of the substance of 

 the fabric, instead of lying upon its free surfaces and being continually 

 cast off from them. Still their individual history is much the same as 

 that of the cells already noticed ; and they differ chiefly in regard to the 

 destination of their products. The first group of this class deserving a 

 separate notice, is that which effects the introduction of aliment into the 

 body ; of those kinds of aliment, at least, which are not received in solu- 

 tion by any more direct means. Along the greater part of the intestinal 

 tube, from the point at which the hepatic and pancreatic ducts enter it, 

 to the rectum, we find the mucous membrane furnished with a vast num- 

 ber of minute tufts or folds, by which its free surface is vastly extended ; 

 these are termed villi. They may be compared to the ultimate root- 

 fibres of trees, both in structure and function ; for each of them gives 

 origin to a minute lacteal or chyle-absorbing vessel, which occupies its 

 centre ; whilst it also contains a copious network of blood-vessels (Fig. 

 10, p. 127), which appears likewise to participate in the act of absorp- 

 tion, by taking up substances that are in complete solution. Now at the 

 end of every villus, there may be seen, whilst the process of digestion 

 and absorption is going on, a cluster of minute opalescent globules, in 

 the midst of which the origin of the lacteal is lost. These globules, 

 whose size varies from l-1000th to l-2000th of an inch, are composed 

 of a milky fluid, which is evidently the same with that which is found in 



