42 THE PINEAL ORGAN 



all the available inner intermediate zone has been utilized in the formation 

 of the anterior end of the embryo, the remaining parts of the outer zone. 

 st will come into apposition and fuse so as to form the right and left 

 halves of the single body and tail. 



A modification of Rauber's theory was later described by Hertwig, 

 who pointed out that Rauber was wrong in considering the whole extent 

 of the germinal ring as constituting the boundary of the blastopore, and 

 that multiple neurulae developed from a single gastrula. Hertwig, on 

 the contrary, believed that multiple growths develop from several gastrular 

 invaginations, in confirmation of which standpoint he cited the discovery 

 of double gastrula? in Amphioxus by Wilson and the observations of 

 Schmidt, who demonstrated the existence of two separate gastrular 

 invaginations in the germinal discs of young trout ova. In explanation 

 of his theory he suggested the modification of Rauber's theory which is 

 illustrated in Fig. 31, D, E, F. In D he shows two gastrular invaginations, 

 k 2 , k l close together and growing inward from the margin of the germinal 

 disc. Between them is a short " inner intermediate zone " as described 

 by Rauber, and the rest of the circumference corresponds to Rauber's 

 " outer intermediate zone." In E it is seen how, as the disc enlarges, the 

 adjoining parts of the ring are gradually transformed into the " primitive 

 mouth " (or, in other words, become included in the boundaries of the 

 two blastoporic openings which now open by a single common orifice). 

 It is obvious that the nearer the embryonic rudiments or axes are to each 

 other, the sooner will the inner intermediate zone be used up in the forma- 

 tion of the embryo, and the right and left halves of the outer intermediate 

 zone come into contact ; and that as a result the two primarily separate 

 gastrular cavities will form posteriorly a single common gastrular cavity, 

 as shown in E and F. In the further course of development the margins 

 of the blastopore or primitive streak can only be increased by additions 

 from the lateral intermediate zones, which meet one another in the 

 median plane, as in the formation of a normal single embryonic axis. 

 Hertwig further emphasized the important point that multiple formations 

 probably arise at the time of gastrula-formation, or even before the time 

 when a gastrula is actually present. 



Fischel also criticized Rauber's " radiation theory " ; he recognized 

 that a generalization of this theory was in many instances met by grave 

 difficulties, and further that it was even insufficient to explain the develop- 

 ment of a Duplicitas anterior, for which Rauber had first postulated the 

 theory as affording a solution. Making use of the schematic diagrams, 

 Fig. 32, A, B, C, which were employed by Kopsch to illustrate the develop- 

 ment of Duplicitas anterior according to the " concrescence theory " of 

 His (see legend), Fischel and Kopsch point out that only a very small part 



