GENERAL CONSIDERATIONS 281 



2. Early History of the Cleavage (Cell-division) Concept 



An initial appreciation of the role and importance of the cell in embryonic 

 development was awakened during the middle period of the nineteenth cen- 

 tury. It really began with the observations of Prevost and Dumas in 1824 on 

 the cleavage (segmentation) of the frog's egg. The latter observations repre- 

 sented a revival and extension of those of Swammerdam, 1738, on the first 

 cleavage of the frog's egg and of Spallanzani's description in 1780 of the first 

 two cleavage planes, "which intersect each other at right angles," in the egg 

 of the toad. Other studies on cleavage of the eggs of frogs, newts, and various 

 invertebrates, such as the hydroids, the starfish, and nematodes, followed the 

 work of Prevost and Dumas. The first reported cleavage of the eggs of a rabbit 

 was made in 1838-1839, a fish in 1842, and a bird in 1847. (See Cole, '30, 

 p. 196.) Newport, in 1854, finally founded the new preformation by showing 

 that the first cleavage plane in the frog's egg coincided with the median plane 

 of the adult body (Cole, '30, p. 196). 



In the meantime, the minute structures of the bodies of plants and animals 

 were intensively studied, and in 1838-1839, the basic cellular structure of 

 living organisms was enunciated by Schleiden and Schwann. Following this 

 generalization, many studies were made upon the phenomenon of cell division 

 in plant and animal tissues. These observations, together with those made 

 upon the cleaving egg, established proof that cells arise only by the division 

 of pre-existing cells; and that through cell division the new generation is 

 formed and maintained. Thus it is that protoplasm, in the form of cells, as- 

 similates, increases its substance, and reproduces new cells. Life, in this 

 manner, flows out of the past and into the present, and into the future as a 

 never-ending stream of cellular substance. This idea of a continuous flow 

 of living substance is embodied well in the famed dictum of R. Virchow, 

 "Omnis Cellula e Cellula," published in 1858 (Wilson, E. B., '25, p. 114). 



The consciousness of life at the cellular level acquired during the middle 

 period of the nineteenth century thus laid the groundwork for future studies 

 in cytology and cellular embryology. Much progress in the study of the cell 

 had been made since R. Hooke, in 1664, described the cells in cork. In 

 passing, it should be observed, that two types of cell division, direct and 

 indirect, were ultimately defined. For the latter, Flemming in 1882, proposed 

 the name mitosis, while the direct method was called amitosis. 



3. Importance of the Cleavage-Blastular Period of 

 Development 



The period of cleavage and blastular formation is a time of profound dif- 

 ferentiation as well as one of cell division. For, at this time, fundamental 

 conditions are established which serve the purposes of the next stage in de- 

 velopment, namely, gastrulation. Experimental embryology has demonstrated 



