CHAPTER III 

 THE ONTOGENESIS OF VERTEBRATES 



"... the embryological record, as it is usually 

 presented to us, is both imperfect and misleading. It 

 may be compared to an ancient manuscript, with many 

 of the sheets lost, others displaced, and with spurious 

 passages interpolated by a later hand. . . . Like 

 the scholar with his manuscript, the embryologist has 

 by a process of careful and critical examination to 

 determine where the gaps are present, to detect the 

 later insertions, and to place in order what has been 

 misplaced." 



FRANCIS BALFOUR, Comparative Embryology. 



Vol. I, p. 3. 



WITH the exception of a few cases of asexual reproduction, 

 that is, cases in which an individual arises from a single parent, 

 every multicellular organism results from a conjugation be- 

 tween a macro- and a micro-gamete. These are called the 

 ovum and spermatozoon, respectively, and are the product of 

 two distinct parent individuals. Precisely the same phenome- 

 non occurs frequently among colonial unicellular organisms, 

 where an entire colony produces gametes of only one sort, 

 and in this case the distinction between such a colony and 

 the mass of cells which constitute the body of a simple 

 Metazoan is extremely slight and depends solely upon the 

 amount of differentiation between the individual cells and the 

 consequent degree of mutual interdependence attained. In 

 both cases the cell mass, aside from the gametes, constitutes a 

 soma, composed in the one case of homogenous, in the other 

 of heterogenous, cells. The soma, or cell colony, is perishable 

 and restricted to a definite time of existence; the gametes by 

 their conjugation produce zygotes, each of which, by its re- 

 peated division, may form a new soma, that is, the colony, or 

 the individual, of the succeeding generation. 



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