GENERAL EMBRYOLOGY 149 



divisions of the Protozoa — cell divisions, and tliose of tlie Metazoa — divisions 

 of cell complexes, are brought into the same category, in spite of their different 

 morphological value. Further, they accept a causal connection vith fertiliza- 

 tion in a form that does not agree with the actual conditions. 



To make this clear we start with the fact that every reproduction depends 

 upon an increase of cells. In unicellular organisms reproduction and multi- 

 plication of cells are one and the same thing. In multicellular organisms, on 

 the other hand, two Icinds of cell division may be distinguished, which may be 

 called somatic and propagating cell divisions. Both are continuations of the 

 cleavage process. The first is concerned with the growth of the individual, 

 since it increases the formative material for the functioning organs; the other 

 renders reproduction possible, since it furnishes the sex cells from which the 

 new individual develops. 



Fertilization' enters in the course of the cell divisions which occur during life, 

 in the Protozoa as the fusion of two individuals, in the Metazoa as the union of 

 two sex cells which contain the anlage of the new individual. Both have in 

 common the union (amphimixis) of two organizations to a combined organ- 

 ization. In the Protozoa the influence which fertilization exerts upon repro- 

 duction is very different in the different classes. In the Infusoria the rate of in- 

 crease is not altered to any appreciable extent; in the Rhizopoda it is brought to 

 a standstill, in which a period of rest (encystment) ensues; in Sporozoa it leads 

 to a special form of increase and only in this group is there a sexual reproduction. 

 The two others show that fertilization and reproduction are not necessarily 

 connected, that the most important result of fertilization is not that it renders 

 increase possible, but that, in some way not yet clear but still fundamentally 

 important, it modifies the organism and influences the vital processes. 



With these conditions an explanation is necessary of why, in the Metazoa 

 in contrast to the Protozoa, there is such an intimate relation of fertilization to 

 reproduction. It was held, indeed is the prevailing idea, that fertilization was 

 introduced to render reproduction possible. The explanation lies in multi- 

 cellularity. If it be necessary for the maintenance of the organism that the char- 

 acters of two organizations be fused in one, the fusion can only take place when the 

 characters of each or their anlagen are condensed into a single cell, as is the case 

 ^^'ith egg reproduction. But the union of two multicellular organisms to a 

 single one is usually but a juxtaposition of parts; in only a few cases, not clearly 

 understood (graft hybrids of many plants), is there a mingling of characters. 

 Hence the inherited reproduction of the colonial Protozoa, which always 

 arise from a single cell, is retained and combined with fertilization. On the 

 other hand, the division and budding of the jNIetazoa are new acquisitions which 

 have arisen independently in the different groups in adaptation to special con- 

 ditions of life. They may be traced back to the phenomena of regeneration, to 

 the capacity of parts of an organism to re-form the whole. While typical re- 

 generation results from a loss of a part from external causes, division and bud- 

 ding are apparently spontaneous, the result of the growth of the organism. 

 That asexual (better 'propagative') reproduction occurs chiefly in lower animals 

 is not a sign of its primitive character, but only the result of the fact that only 

 in the simpler organisms is there the capacity of complete regeneration. This 

 explains why the striking regularity which prevails in sexual reproduction is 

 lacking in regeneration, division and budding. 



5. The Different Forms of Sexual Development. 



Embryonic and Postembryonic Development. — While the occur- 

 rences described (fertilization and cleavage of the egg, formation of the 



