ORIGIN OF MONSTERS 533 



latter and the chemically destroyed one is loosened more and 

 more until it is lost entirely and the altered fragment breaks off. 

 This will at once become apparent if we recall the facts regard- 

 ing the formation of the embryo in teleosts so well established by 

 the investigations of Kopsch ('96, '04). According to this author 

 the formation of the embryo comes about in the following 

 manner ('96, p. 120) : 



An dem zelligen Randring (der Keinischeibe 24 Stunden nach der 

 Bildiing des ersten Umschlages) muss man zwei Bezirke unterscheiden. 

 Einen embryobildenden und einen nicht (direct) embryobildenden. 

 An dem embryobildenden Bezirk, welcher an der Stelle der ersten 

 Einstiilpung gelegen ist, haben wir weiter zu unterscheiden einen der 

 Medianlinie naher gelegenen Teil, dessen Zellen den Kopf des Embryos 

 bilden. und jederseits lateral von diesem Bezirke Zellengruppen, welche 

 im Laufe der Entwickelung in der Medianlinie zusammenkommen, 

 und den Knopf bilden. Der Knopf stellt ein Wachstumcentrum vor, 

 von welchem Rumpf und Schwanz gebildet werden, wobei Zellen des 

 nicht direct zum Aufbau des Embryos verwendeten Teiles des Rand- 

 ringes im Laufe der Umwachsung des Betters zum Knopf gelangen 

 und dort ebenfalls zur Bildung des Embryos benutzt werden. 



In both the area which is to form the head (K) and in the 

 parts which are to form the 'Knopf (figs. 50 and 51, p. 534) the 

 organs of the head and trunk are potentially contained. For 

 experiments of Kopsch have shown that if parts of any one 

 of these areas be destroyed the organs which these eliminated 

 fragments potentiallv represent, will be lacking (Kopsch '96, 

 p. 121). 



Fully in accord with these data is the \T.ew which I have 

 expressed above, namely that terata of the head owe their 

 origin to blastolytic injury at the anterior part of the area which 

 is to form the head. This area would correspond to the area K 

 in Kopsch's diagram and the blastolytic elimination of an antero- 

 median chemically incapacitated fragment of this area must be 

 assumed as taking place before the 'Knopf has yet been formed, 

 or thereabout. The eliminated fragment would contain parts 

 of the material potentially representing maxillary and mandibu- 

 lar arches, the olfactory pits, the tissues of the interocular area 

 and very often more or less of the ophthalmoblastic material. Fol- 



