343 



In 1853, Newport published the results of his observations on 

 the position of the embryo of the Frog with reference to certain cleav- 

 age grooves. I quote the author's words: "Repeated observations have 

 shown that in one part of the yolk the head and in another part the 

 body and tail of the forthcoming being begin. The appointment of the 

 parts is further indicated, very early, by conditions in the yolk cleavage 

 . . . conditions so constant that I am able to predicate shown after the 

 completion of the horizontal cleft, or at the beginning of the fourth 

 change, where the head and where the tail of the future being will 

 be placed .... When this stage of the segmentation of the yolk is 

 arrived at, the position of the body and head of the coming being can 

 be determined with certainty, so that it is not necessary to follow 

 further the changes during segmentation. If the cell be marked op- 

 posite the first commencing post crucial subdivisions (third verticals) and 

 then set aside for the formation of the embryo, the trunk and tail of 

 the developing being will be found to originate in this subdividing 

 part behind the crucial sulcus (second vertical) and the head to be 

 produced in the part on the other side, or in front of the sulcus in 

 which the secondary segmentation (third verticals) last appears." 



Pflüger's careful observations and experiments led him to con- 

 clude that the embryo developed in the white (lower) hemisphere as 

 stated in these words: "Was mir auch dieser Versuch mit der äußersten 

 Entschiedenheit zeigte, war die Entwickelung des Rückenmarks, sowie 

 wohl auch der Medulla oblongata aus der Substanz der weißen Hemi- 

 sphäre." 



It is especially noteworthy that the author admits the possibility 

 of the neural plate, containing the basis of the brain and the upper 

 portion of the cord, being derived from the darker hemisphere, viz: 

 "Um nicht mißverstanden zu werden, möchte ich hervorheben, wie ich 

 keineswegs bewiesen zu haben glaube, daß die ganze Uranlage des 

 centralen Nervensystems ein Derivat der weißen Hemisphäre des Eies 

 sei .... so bleibt es denkbar, daß die vorderen Teile der Markanlage, 

 die dem Gehirn und möglicherweise sogar dem oberen Teil des 

 Rückenmarks entsprechen, sich in der schwarzen Hemisphäre bilden." 



The researches of 0. Schultze led him to regard the older view 

 of VON Baer as correct, viz: "Vielmehr ist die alte von BAER'sche 

 Anschauung, der sich fast alle Embryologen angeschlossen haben, voll- 

 kommen richtig, indem die Eiachse, vom schwarzen zum weißen Pol ge- 

 rechnet, die dorsoventrale Richtung des späteren Tieres angiebt und 

 das Centralnervensystem sich nur auf der dunklen animalen Hälfte ent- 

 wickelt." 



