148 EMBRYOLOGY. 



indeed he derives it exclusively from the lower germ-layer by a 

 process of fission. He designates the three layers as the upper or 

 sensorial, the middle or motor-germinative, and the lower or trophic. 

 The four secondary germ-layers of VON BAER come into existence 

 subsequently by a repetition of the fission, whereby the middle germ- 

 layer is split, at least in its lateral portions (lateral plates), into the 

 dermo-fibrous layer and the intestino-fibrous layer (Hautfaser- und 

 Darmfaserblatt), between which arise the thoracic and body-cavities. 



EEMAK in his account approximates the true state of affairs, as 

 detailed in the preceding chapters, more nearly than VON BAER ; 

 however, both made the same mistake of interpreting the formation 

 of the germ-layers as always a process of disassociation or fission. 

 That is also the rock on which were wrecked the researches of numer- 

 ous other investigators, who in the decennary succeeding REMAK 

 dealt with the important question of the origin of the germ-layers. 

 It was difficult to decide this question for the higher Vertebrates, 

 which have been most frequently investigated ; so that very contra- 

 dictory opinions were expressed relative to the development of the 

 middle layer whether it was exclusively from the lower (REMAK), 

 exclusively from the upper, or from both layers. 



This question could be clearly understood only upon the establish- 

 ment of new general standpoints. These could be acquired only by 

 the comparative method, and by the study of lower Vertebrates and 

 the Invertebrates. 



Two fundamental processes needed to be better comprehended: 



(1) How are the two primary germ-layers developed-? 



(2) How are the two middle germ-layers developed ? 



By means of the comparative developmental method, one question 

 has been brought nearer to a solution in tlie gastrcea-theory, the other 

 in the codom-theory. 



In the study of the first problem, which was the earlier solved, 

 HUXLEY and KOWALEVSKY, HAECKEL and RAY LANKESTER, have 

 shown especial merit. They demonstrated, partly through anato- 

 mical, partly through embryological studies, that, with the exception 

 of the Protozoa, the body of every invertebrated animal is constructed 

 of layers, which may be compared with the primary germ-layers of 

 Vertebrates. 



The highly gifted English zoologist HUXLEY distinguished as early 

 as the year 1849 two membranes in the Medusae, an outer and an 

 inner layer, out of which alone their bodies are constructed ; and at 

 the same time expressed the happy idea that physiologically they 



