REPRODUCTION. 53 



embryology, or the history of the origin and development of 

 tissues and organs, will occupy a prominent place in the vari- 

 ous chapters of this work. The student will, therefore, at the 

 outset be furnished with a general account of the subject, while 

 many details and applications of principles will be left for the 

 chapters that treat of the functions of the various organs of 

 animals. The more knowledge the student possesses of zoology 

 the better, while this science will appear in a new light under 

 the study of embryology. 



Animals are divisible, according to general structure, into 

 Protozoa, or unicellular animals, and Metazoa, or multicellular 

 forms — that is, animals composed of cell aggregates, tissues, or 

 organs. Among the latter one form of reproduction appears 

 for the first time in the animal kingdom, and becomes all but 

 universal, though it is not the exclusive method ; for, as seen in 

 Hydra, both this form of generation and the more pirimitive 

 gemmation occur. It is known as sexual multiplication, which 

 usually, though not invariably, involves conjugation of two un- 

 like cells which may arise in the same or different individuals. 

 That these cells, known as the male and female elements, the 

 ovum and thpi ap grma.toznon , are not necessarily radically dif- 

 ferent, is clear from the fact that they may arise in the one in- 

 dividual from the same tissue and be mingled together. These 

 cells, however, like all others, tell a story of continual progress- 

 ive differentiation corresponding to the advancing evolution of 

 higher from lower forms. Thus hermaphroditism, or the coex- 

 istence of organs for the production of male and of female cells 

 in the same individual, is confined to invertebrates, among 

 which it is rather the exception than the rule. Moreover, in 

 such hermaphrodite forms the union of cells with greater differ- 

 ence in experiences is provided for by the union of different in- 

 dividuals, so that commonly the male cell of one individual 

 unites with (fertilizes) the female cell of a different individual. 

 It sometimes happens that among the invertebrates the cells 

 produced in the female organs of generation possess the power 

 of division, and continued development wholly independently 

 of the access of any male cell {parthenogenesis) ; such, how- 

 ever, is almost never the exclusive method of increase for any 

 group of animals, and is to be regarded as a retention of a more 

 ancient method, or perhaps rather a reversion to a past biologi- 

 cal condition. No instance of complete parthenogenesis is 

 known among vertebrates, although in birds partial develop- 



