xUv THE ANIMAL KINGDOM. 



those of dull intellect are led or excelled by those of greater intelligence, and this 

 indicates that they are not simple automata, but are also in a degree, or within their 

 own sphere, free agents. 



REPRODUCTION AND EMBRYOLOGY. 



On a jjrevious page, as an introduction to the subject of tissues, we discussed some 

 of the earlier features of the development of animals, but now we need, in a brief 

 manner, to consider the subject from another point of view, and, in a general way, to 

 trace it out for the whole animal kingdom. 



As soon as the microscope was perfected and constantly used by morphologists as 

 an instrument of exact research, a flood of light was thrown upon the subject of repro- 

 duction. In its ultimate analysis reproduction essentially consists in the separation of 

 a portion of an adult animal from itself, this portion developing into an animal like the 

 parent. It was seen that in the one-celled animals this process was identical with cell 

 division, and it was called fission. It was also found that, as in the Hydra and allied 

 polyps, a bud would form, and develop into a Hydra, and finally separate from the 

 original parent Hydra. Although in the first place the formation of the bud is due 

 to cell-division, a single cell giving rise to the bud, this process is called budding or 

 gemmation. It was likewise discovered that in those animals which produce eggs, the 

 latter were fertilized by a very minute and greatly modified cell, called the sperm-cell, 

 or spermatozoon. 



Now, in most of the many-celled animals there are two kinds of individuals, one 

 female, which produces in its ovaries eggs, and the other the male, which produces in 

 its testes the spermatozoa. Reproduction, or fertilization of the egg, in such animals, 

 consists in the fusion of the sperm-cell with the nucleus of the egg ; this is called sex- 

 ual reproduction. From the moment of fertilization begins the life of the germ, 

 which is called an embryo, while the history of the changes undergone by the embryo 

 from the time of fertilization of the egg to maturity is called Embryology. 



As was said a few pages back, the egg is essentially a simple cell, and in its earlier 

 condition it is not to be distinguished from the other cells of the rejiroductive organs, 

 but with development it changes in many resjjects, prominent among which is an in- 

 crease in size. The essential part of the egg is its protoplasm, but to this is usually 

 added a varying quantity of a nutritive material, the deutoplasm or food-yolk. Besides, 

 in most forms, protective envelopes, etc., are added. The most familiar egg, that of 

 the barnyard fowl, is poorly adapted to give us an idea of the true nature of an egg. 

 Here the protojilasm is very small in quantity, and forms but a small ]iatch on one side 

 of the ' yolk,' which is almost entirely jirotoplasm. Another adventitious substance is 

 the ' white,' while the shell and the membranes are merely protective, and not essential 

 features. In another respect this egg is unsuited for our purposes, for, at the time of 

 laying, the segmentation has jirogressed to a considerable extent, and the egg is no 

 longer to be regarded as a simple cell. 



The tyjiical egg, then, is a mass of protoplasm, which is differentiated, as in any 

 other cell, into nucleus and nucleolus, the latter in turn exhibiting a structure to be de- 

 scribed below. In almost all eggs there is found one or more protective envelopes, 

 which, according to the mode of origin, have received different names. When it is 

 produced by the egg itself, it is called the vitelline membrane ; when by the ovarian 

 tissues of the parent it receives the name chorion. These envelopes in many forms 



