208 REGENERA TION 



a tube. He thought it probable that other embryonic organs might 

 arise in the same way. His views made at the time no impres- 

 sion on his contemporaries, and lay buried until 1812, when Meckel 

 republished Wolff's work in a German translation. Pander, in 1817, 

 distinguished two layers in the early embryo, a serous and a mucous, 

 and stated that later a third, vascular layer appears between the 

 other two. Von Baer published in 1829 his celebrated memoir on 

 the development of the chick, in which he made out two primary 

 layers in the germ, the animal and the vegetative layer, and held that 

 each of these separates into two to produce the four embryonic 

 layers. Remak, in 1851-1855, gave a more precise description of the 

 germ-layers, and stated that from the innermost layer, the epithelium 

 and glandular cells of the digestive tract arise (including the lining 

 of the glands that open into the digestive tract). From the outer- 

 most layer he showed that the integument and sense organs and the 

 nervous system develop, and from the two middle layers develop the 

 muscles, blood, excretory, and reproductive organs. By the term 

 " germ-layers " was meant at this time only that the embryo is formed 

 out of sheets. 



Huxley in 1849 pointed out that a medusa is made up of two 

 layers, an outer and an inner, and called attention to their possible 

 equivalency to von Baer's serous and mucous layers. This idea of a 

 resemblance between the layers of an embryo and of an adult of 

 a lower form furnished the starting-point for the more modern for- 

 mulation of the germ-layer hypothesis. Kowalevsky's work on the 

 development of a number of the lower animals showed that there is 

 present in many forms a two-layered stage, or gastrula, formed by 

 an in-turning of the wall of the hollow blastula. In this way two 

 germ-layers are established, an outer and an inner, that correspond 

 to the ectoderm and to the lining of the digestive tract, or endoderm. 

 While Kowalevsky's work did much toward laying the foundation of 

 the modern study of embryology, he himself indulged in very little 

 of the sort of speculation that came into vogue a few years later. 

 Kowalevsky's discovery of the gastrula stage in the embryos of many 

 different groups has been fully confirmed and extended, but the elabo- 

 rate speculations that have been built up on this as a basis have 

 gone far beyond the evidence, and, for a time, drew the attention of 

 embryologists away from more important problems. Haeckel took a 

 more extreme position than most of his contemporaries, and assumed 

 that the gastrula stage that occurs in so many of the groups of meta- 

 zoa corresponds to an ancestral, two-layered adult animal, the gas- 

 traea, from which all the higher forms have descended. The presence 

 of the gastrula in the development was interpreted as a " repetition " 

 of this ancestral adult stage. Thus the two primary layers are sup- 



