THE GERM-LAYERS IN REGENERATION 2CX) 



posed to have an historical meaning. 1 Embryologists soon began a 

 search for a similar mode of interpreting the middle germ-layer, or 

 layers, which led, amongst other views, to the formulation of the 

 "gut-pouch hypothesis." From this point of view the body cavi- 

 ties, or ccelomes, are supposed to have been originally sac-like out- 

 growths from the digestive tract of an ancestral adult animal. Later, 

 these coelome sacs are supposed to have been shut off from connec- 

 tion with the digestive tract their cavities becoming the body cavi- 

 ties, and their walls giving rise to the mesodermal organs. The 

 formation of pouches from the walls of the archenteron of the embryo 

 in several groups of animals has been interpreted as a repetition of 

 the ancestral adult animal. 



A comparison of the germ-layers in different forms very soon 

 led to an attempt to " homologize " the layers in different animals. 

 If the layers have had historically the same origin, or appear in the 

 same way in the embryos, or give rise to the same organs, they are 

 said to be homologous. In the absence of a knowledge of the first 

 two of these conditions it is generally considered sufficient, if it can 

 be shown that similar organs arise from a layer, to " homologize " 

 that layer in the two forms. The study of embryology soon became 

 a search for homologies. The results led to inextricable difficulties 

 and innumerable contradictions until, a reaction setting in, many 

 embryologists became sceptical in regard to the value of this entire 

 method of study. 



The results of a detailed study of the process of cleavage in a 

 number of groups have helped, perhaps, to clear the way for a sounder 

 conception. It has been found that the cleavage of the egg in mem- 

 bers of the groups of annelids, mollusks, and turbellarians is ex- 

 tremely similar so similar, in fact, that it seems hardly possible that 

 they could be due to chance, especially as the series of cleavages is 

 quite complicated. The discovery of these similarities led at once 

 to comparison, and comparison to the establishment once more of 

 homologies, and the homologies led again to contradictions, until at 

 present scarcely any two workers agree as to a criterion of homol- 

 ogy. 2 Leaving this question aside, however, and fixing our attention 

 only on the similarity of the process of cleavage, we are justified, I 

 think, in looking for an explanation of the similarity in some sort of 

 an historical connection. We can eliminate, I think, without discus- 

 sion the possibility of this type of cleavage representing an ancestral 



1 I have given elsewhere (The International Monthly, March, 1901) a fuller treatment 

 of the gastnea theory from the historical point of view. 



2 It may be pointed out that there may he really several kinds of homology, such as 

 homology due to similar origin of the blastomeres, or to their position, or to their fate, etc. 

 The confusion that has arisen may in part result from the attempt to make homologous 

 parts agree in all points. 



P 



