380 THE FORMS OF TISSUES [ch. 



and animals. In other words they tally with, and they further 

 extend, the general and fundamental conclusions laid down by 

 Schwann, in his MikrosJiopische Untersuchungen iiher die Ueherein- 

 stimmung in der Struktur und deni Wachsthuni der Thiere und 

 Pflansen. 



But now that we have seen how a certain limited number of 

 types of eight-celled segmentation (or of arrangements of eight 

 cell-partitions) appear and reappear, here and there, throughout 

 the whole world of organisms, there still remains the very important 

 question, whether in each particular organism the conditions are 

 such as to lead to one particular arrangement being predominant, 

 characteristic, or even invariable. In short, is a particular arrange- 

 ment of cell-partitions to be looked upon (as the pubUshed figures 

 of the embryologist are apt to suggest) as a specific character, or 

 at least a constant or normal character, of the particular organism ? 

 The answer to this question is a direct negative, but it is only in 

 the work of the most careful and accurate observers that we find 

 it revealed. Rauber (whom we have more than once had occasion 

 to quote) was one of those embryologists who recorded just what 

 he saw, without prejudice or preconception; as Boerhaave said 

 of Swammerdam, quod vidit id asseruit. Now Rauber has put on 

 record a considerable number of variations in the arrangement of 

 the first eight cells, which form a discoid surface about the dorsal 

 (or "animal") pole of the frog's egg. In a certain number of 

 cases these figures are identical with one another in type, identical 

 (that is to say) save for slight differences in magnitude, relative 

 proportions, or orientation. But I have selected (Fig. 168) six 

 diagrammatic figures, which are all essentially different, and these 

 diagrams seem to me to bear intrinsic evidence of their accuracy : 

 the curvatures of the partition-walls, and the angles at which 

 they meet agree closely with the requirements of theory, and when 

 they depart from theoretical symmetry they do so only to the 

 slight extent which we should naturally expect in a material and 

 imperfectly homogeneous system*. 



* Such preconceptions as Rauber entertained were all in a direction likely to 

 lead him away from such phenomena as he has faithfully depicted. Rauber had 

 no idea whatsoever of the principles by which we are guided in this discussion, 

 nor does he introduce at aU the analogy of surface-tension, or any other purely 

 physical concept. But he was deeply under the influence of Sachs's rule of rect- 



