GENERAL INTRODUCTION. 



The Germ-cells. Ovum and Spermatozoon. — Since the establishment of the cell-theory by Schleiden and Schwann in 

 1S38-40, the animal ovum has been recognized as being morphologically a single cell, consisting essentially of a mass of 

 protoplasm (cytoplasm) containing a nucleus, and hence morphologically equivalent to any one of the tissue-cells of which 

 the body is composed. The multicellular body is derived from the ovum by a series of successive divisions or cleavages, 

 the egg-cell dividing into two, four, eight, and so on in more or less regular geometrical progression, until a very large 

 number of cells are produced (Text-fig. I. A-L).' These cells, known in their earlier stages as blastomcrcs, are ultimately 

 differentiated into the elements of the tissues, and among their descendants a certain number assume the character of the 

 original egg-cell, are converted into ova, and thus form the point of departure for the following generation. Every ^^^ is 

 therefore derived by a continuous and unbroken series of cell-divisions from the egg of the preceding generation, and so on 

 backward throughout all preceding generations; it is normally destined to form the first term in a series of cell-divisions 

 extending indefinitely forward into the future. 



In some exceptional cases {^parthcnogcucsis), the ^^^ is capable of initiating this series of cell-divisions without 

 the influence of a male element. In sexual reproduction, howe\-er, which includes all ordinary cases, both among 

 plants and animals, the egg is incapable of division until it has been fertilized, i.e., acted on by an element derived from 

 the opposite sex and known as the spcrin-ccll or spermatozoon. The spermatozoon differs very widely from the ovum in 

 appearance, being extremely minute and provided, in most cases, with a long vibratile tail or flagellum (Text-fig. II. A), 

 bv means of which it swims rapidly about. For this latter reason it was long regarded as a parasitic animalcule or 

 infusorian. Not long after the promulgation of the cell-theory, however, it was sho\\'n that the spermatozoon, like the 

 ovum, is a single cell, consisting of nucleus and cytoplasm, and that it has a like origin, being derived by division from 

 cells pre-existing in the parent body. Inheritance is therefore effected in both sexes by means of cells, and the mechanism 

 of hereditary transmission is to be sought in cell-structure. 



Fertilization. — Broadly speaking, fertilization consists in the union of a single spermatozoon with a single ovum, after 

 which the process of division or cleavage immediately begins. It is true that in many cases — for example, in the shark, the 

 butterfly, the earthworm, the newt — two or more spermatozoa may enter the egg. All the evidence goes to show, however, 

 that even in this case, if development be normal, only one spermatozoon plavs an active part, while the others are passive 

 and sooner or later perish and are absorbed. The fertilized ovum, or oosperm, is therefore the result of the fusion of two 

 germ-cells derived from the two respective se.xes. And since each parent contributes a single germ-cell only to the 

 formation of the embryo, it follows that a single cell is capable of carrying with it the potential sum total of hereditary 



' Text-figures are numbered throughout with Roman numerals to distinguish them from the phototypes. 



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