480 ZOOLOGY SINCE DARWIN. 



The tissues of the body, such as muscle, nerve, and connective tissue, 

 indeed the cells of a single organ acting under separate functional con- 

 ditions, as those of glands, were separated from each other in the 

 microscopical field by means of newly invented methods of staining and 

 impregnation. The old methods of teasing and compressing tissues 

 were superseded by the microtome, which has become indispen sable in 

 these investigations because it enables us to divide an animal into an 

 unbroken series of extremely thin slices. Since by such means one 

 can again restore after section the structure of objects even as to their 

 most elementary parts, we obtained the means of investigating the 

 intimate structure of animals whose small size had hitherto made it 

 impossible to dissect them with the scalpel and whose opacity pre- 

 vented us from obtaining any information concerning them by the 

 compression method. A great advance was also made in the efficiency 

 of the microscope by the construction of apochromatic lenses. 



These all redounded to the benefit of morphology and at no previous 

 period of zoology were there produced so many comprehensive and 

 exhaustive monographs on zootomic and embryological subjects. The 

 purpose of most of these was the establishment of the ancestral tree. 

 But while comparative anatomy was of the first importance in the 

 elucidation of the relationships of existing living forms, embryology 

 lent itself particularly to the task, because it assisted to fill the gaps in 

 the paleontogical record, by comparing the older stages of animal life 

 with the phases of individual development. One result of this activity 

 was the theory of the homology of the germinal layers. As certainly as 

 the egg of all animals has the morphological value of a cell and points 

 to unicellular primitive beings (protozoa) as the beginning of all higher 

 organization, so surely must also the subsequent embryonal stages of 

 multicellular animals (metazoa) afford reminiscences of common ances- 

 tral forms. And Haeckel, starting from this premise, believed that 

 in the "gastrula" developmental stage — a widely extended form con- 

 sisting of two concentric cell layers and a primitive mouth — he had, in 

 fact, found the type of that ancestral form common to all multi cellular 

 animals (the Gastraea). 1 Its two cell layers must necessarily have the 

 same morphological value in all, and thereby was presented the pos- 

 sibility of tracing the homology of organs, not only in single stocks, 

 but also throughout the entire kingdom of the metazoa. 



But when we examine the facts cited in support of this wide-reach- 

 ing generalization, we are obliged to admit that it has not as yet been 

 established. The more profound our embryological knowledge becomes, 

 and the more exact our comparative researches are, the higher loom 

 the difficulties. But the essential trouble lies not so much in the diffi- 

 culty of readily bringing under the gastrula scheme all the observed 



'E. Haeckel, "Die Gastra^a-Theorie, die phylogenetische Classification des Thier- 

 reichs und die Homologie der Keimblatter" (Jenaische Zeitschrift fur Naturwissen- 

 schat't, VIII. Bd., Jena, 1874). 



