70 EMBRYOLOGY. 



EOSENHOF, it was PREVOST ET DUMAS who were the first to describe, in 1824, 

 the manner in which regular furrows arise on the Frog's egg, and how by 

 means of these the whole surface is divided into smaller and smaller areas. 

 According to the French investigators, the furrows were restricted to the sur- 

 face of the egg. However, only a few years later, RUSCONI (1826) and C. E. 

 V. BAER recognised that the furrows visible at the surface correspond to 

 fissures which extend through the whole mass of the yolk, and divide it into 

 separate parts. Even in his time VON BAER rightly characterised the whole 

 process of segmentation, in which he discerned the first impulse of life, as an 

 automatic division of the egg-cell, but subsequently he abandoned this, the 

 right path, since he sought for the meaning or! division in the dictum : that 

 "all yolk-masses are subject to the influence of the fluid and volatile 

 components of the fertilising material." 



In the next decennary there followed numerous discoveries of the process of 

 segmentation in other animals. During this period acquaintance was also 

 gained with partial segmentation. After KUSCONI and VOGT had seen it in 

 the case of fish eggs, KOLLIKER gave, in the year 1844, the first detailed 

 description of it as seen in the eggs of Cephalopods, and four years later 

 COSTE described it in the Hen's egg. 



The question of the significance of the cleavage-process has engaged the 

 earnest attention of investigators, and has given rise to many controversies. 

 The discussion first took a definite turn upon the establishment of the cell- 

 theory. The question was, to determine whether and in what manner cleav- 

 age was a process of cell-formation. Although there were already many 

 observations on the division of eggs, SCHWANN himself took no definite posi- 

 tion on this question. The views of other investigators were at variance for 

 years. There was a difference of opinion as to whether the egg or the ger- 

 minative vesicle was a cell, whether the segments resulting from cleavage 

 possessed a membrane or not, and whether these segments were to be regarded 

 as cells or not. In the earlier literature the germinative vesicle and the 

 nuclei of the cleavage-spheres were often designated as embryonic cells, and 

 the surrounding yolk-mass as an enveloping sphere. The difficulty of com- 

 prehending the process of segmentation was also aggravated by the false 

 doctrine of free cell -formation from an organic matrix the cytoblastema 

 founded by SCHWANN. It remained for a long time a controverted point 

 whether the tissue-cells of the adult organism were the direct descendants of 

 the segmentation-spheres, or whether they arose at a later period by means 

 of free cell-formation from cytoblastema. After NAGELI on the botanical 

 side had adopted the right course, it was the service of KO"LLIKER, KEICHERT, 

 KEMAK, and LEYDIG to have paved the way to a comprehension of cleavage, 

 and to have shown that free cell-formation does not take place, but that all 

 cellular elements arise in uninterrupted sequence from the egg-cell. 



As far as regards the different kinds of cleavage, KOLLIKER designated 

 them as total and partial. VAN BENEDEN has given in his " Recherches sur 

 la composition et la signification de 1'ceuf " a more exhaustive review of the 

 subject, and has also expounded in a clear way the signification of the 

 deutoplasm for the different kinds of cleavage. Subsequently HAECKEL mate- 

 rially simplified the categories of segmentation recognised by VAN BENEDEN, 

 and proposed in his " Anthropogenic " and in his paper " Die Gastrula und die 

 Eif urchung " the classification of the methods of cleavage on which is based 

 the scheme previously given, and according to which total cleavage is divided 



