CH. IVJ CLEAVAGE OF THE EGG 49 



stages these smaller bodies fuse by a vital process into a new 

 whole, and a new individual is thus produced from the frag- 

 ments of the first. 



Schwann and Schleiden promulgated the cell-theory in 1838- 

 1839. This produced an effect on all subsequent interpreta- 

 tions of the segmentation of the frog's egg. The main points 

 to settle were : first, whether the process of cleavage is a pro- 

 cess of cell-division, i.e. whether the egg is a cell that divides ; 

 second, whether the bodies that result from the segmentation 

 of the egg pass over into the cells of the embryo. The search 

 for the nucleus, before and after the process, also occupied the 

 attention of workers on the subject. Bergmann ('41) was the 

 first to treat the process of cleavage from the cell standpoint. 

 The first divisions of the egg did not produce true cells, he said ; 

 yet as the results of these divisions went over directly into the 

 cells of the embryo, therefore the division of the batrachian 

 egg is the introduction of cell-formation into the yolk. Later, 

 he said that the yolk may be thought of as strongly disposed 

 to form cells, but that nuclei are wanting. Reichert's in- 

 terpretation ('46) was a step backwards. Kolliker, in 1843, 

 described the segmentation-spheres as Avithout a membrane 

 and containing spore-like bodies which multiplied endoge- 

 nously. When these bodies are set free, he thought, they 

 become the cells from which the tadpole is built up. Cramer 

 ('48) thought that the early cleavage-spheres formed mem- 

 branes (cell- walls) and were the progenitors of the true cells 

 of the body. Remak ('50-'55) argued that the cleavage-pro- 

 cess was the beginning of cell-division, and that the products 

 resulting from division formed the cells of the embryo. This 

 statement marked a distinct advance and is the standpoint 

 taken at the present time. Moreover, Remak thought it highly 

 probable that there was a continuity of the original egg-nucleus 

 with the cleavage-nuclei. Max Schultze, in 1863, described 

 admirably the process of cleavage of the frog's egg. He spoke 

 of the egg as a cell with protoplasm and nucleus, and of the 

 process of cleavage as cell-division. Ordinary cell-division 

 depends, he said, on the contractility of the protoplasm. The 

 same property belongs to the egg-yolk, since it divides like a 

 true cell. 



