BIOGENETIC LAW OF EMBRYONIC RECAPITULATION 347 



also have done much to overthrow the concept concerning the rigid specificity 

 of the three primary germ layers of entoderm, mesoderm, and ectoderm. 



D. Introduction of the Words Ectoderm, Mesoderm, Endoderm 



Various students of the Coelenterata, such as Huxley (1849), Haeckel 

 (1866) and Kleinenberg (1872), early recognized that the coelenterate body 

 was constructed of two layers, an outer and an inner layer. Soon the terms 

 ectoderm (outside skin) and endoderm (inside skin) were applied to the outer 

 and inner layers or membranes of the coelenterate body, and the word 

 mesoderm (middle skin) was used to refer to the middle layer which ap- 

 peared in those embryos having three body layers. The more dynamic 

 embryological words epiblast, mesoblast, and hypoblast (entoblast) soon 

 came to be used in England by Balfour, Lankester, and others for the words 

 ectoderm, mesoderm, and endoderm, respectively. The word entoderm is used 

 in this text in preference to endoderm. 



E. Importance of the Blastular Stage in Haeckel's Theory of "The 

 Biogenetic Law of Embryonic Recapitulation" 



In 1859, Charles Darwin (1809-82) published his work On the Origin 

 of Species by Means of Natural Selection. This theory set the scientific world 

 aflame with discussions for or against it. 



In 1872 and 1874, E. Haeckel (1834-1919), an enthusiast of Darwin's 

 evolutionary concept, associated the findings of Kowalewski regarding the 

 early, two-layered condition of invertebrate and vertebrate embryos together 

 with the adult, two-layered structure of the Coelenterata and published the 

 blastaea-gastraea theory and biogenetic principle of recapitulation. In these 

 publications he applied the term gastrula to the two-layered condition of the 

 embryo which Kowalewski has described as the next developmental step suc- 

 ceeding the blastula and put forward the idea that the gastrula was an em- 

 bryonic form common to all metazoan animals. 



In his reasoning (1874, translation, '10, Chap. 8, Vol. I), Haeckel applied 

 the word blastaea to a "long-extinct common stem form of substantially the 

 same structure as the blastula." This form, he concluded, resembled the 

 "permanent blastospheres" of primitive multicellular animals, such as the 

 colonial Protozoa. The body of the blastaea was a "simple hollow ball, filled 

 with fluid or structureless jelly with a wall composed of a single stratum of 

 homogeneous ciliated cells." 



The next phylogenetic stage, according to Haeckel, was the gastraea, a 

 permanent, free-swimming form which resembled the embryonic, two-layered, 

 gastrular stage described by Kowalewski. This was the simple stock form for 

 all of the Metazoa above the Protozoa and other Protista. Moreover, he 

 postulated that the gastrula represented an embryonic recapitulation of the 

 adult stage of the gastraea or the progenitor of all Metazoa. 



