12 GENERAL EMBRYOLOGY 



Reproduction following gamete formation and fusion (syn- 

 gamy) is commonly known as "sexual" reproduction, while the 

 term "asexual" is applied to modes of reproduction which do 

 not involve the fusion of gametes. It is now clear, however, 

 that several different and unrelated processes are included under 

 each of these heads. Thus as " asexual" must be included 

 such diverse modes of propagation as budding, fission, brood 

 formation, parthenogenesis, development from spores, and so 

 forth; and "sexual" reproduction would also follow many 

 forms of syngamic fusion. Two further conditions tend to 

 rob these terms of their precise significance; these are, the 

 existence of transitional conditions, some of which have been 

 mentioned above, and the doubtfully essential character of 

 the primary relation of syngamy to reproduction. These 

 useful terms are to be retained only as convenient though 

 inexact expressions, and are to be used in much the same way 

 that we still employ the convenient words "vertebrate" and 

 "invertebrate." Even so, we may avoid any unintentional 

 implications by substituting the more exact terms amphigony 

 and monogony for sexual and asexual respectively. 



Without suggesting the idea of direct relationship among any 

 existing forms, we may say that it is but a short step from the 

 processes of gamete formation and reproduction among the 

 colonial Protozoa, to the mode of reproduction characteristic 

 of the Metazoa, which after all may be considered highly 

 organized cell colonies. One of the more distinctive as well as 

 more obvious characteristics of the Metazoa is the structural 

 and functional differentiation of large groups of cells as tissues, 

 each variety of tissue performing, in the animal economy, 

 chiefly one function, such as conduction, support, or excretion. 

 Among these various tissues is the reproductive or germinal 

 tissue, which, in all save a few of the lower Metazoa, is always 

 in the form of definite organs, the gonads. The distinction, 

 suggested by some of the colonial Protozoa, between the repro- 

 ductive and the vegetative tissues was probably the earliest of 

 those "physiological divisions of labor" which involved tissue 

 differentiations in the Metazoa. The essential cells of the 



