258 Prof. E. Hackel on the Position of the 



a muscular lamella^ begins to be developed between the two 

 lamella? ; and this constitutes, in the higher Acalephse, a 

 distinct mesoderm with greatly differentiated products. Now, 

 as, according to Kleinenberg's careful exposition, this muscular 

 lamella proceeds directly from the exoderm, and as Kolliker 

 also with great certainty derives the mesoderm of tlie Aca- 

 leplise from the exoderm, the question of the origin of the 

 middle germ-lamella ought by this means to be brought 

 nearer to its solution. It is well known that most ontogenists 

 derive the middle germ-lamella in tlie Vertebrata from the 

 splitting of the inferior one, whilst others make it originate 

 from the superior germ-lamella. The morphology of Hydra, 

 in wliich the individual muscles composing the middle lamella 

 are nothing more than internal processes of the cells of the 

 exoderm, and remain throughout life in connexion therewith, 

 appears to prove the origin of the mesoderm or muscular 

 lamella from the outer germ-lamella, the exoderm (see note 

 p. 261). 



Greater difficulties are presented by the question of the 

 origin of the generative organs. In the embryology of the 

 Vertebrata, the first rudiments of the sexual glands have been 

 derived, even in the most recent times, by some from the 

 upper, by many from the middle, and by others from tlie 

 inferior germ-lamella. Consequently all the three possible 

 views have at present their supporters. If we endeavour to 

 solve these contradictions on the basis of homology above 

 affirmed by regarding the origin of the sexual cells in the 

 Zoophytes as furnishing a rule, we find unfortunately that the 

 same differences prevail here also. Nearly an equal number 

 of observers represent the ova and sperm-cells of the Aca- 

 lephas as produced from the exoderm and from the entoderm. 

 The sexual cells originate from the entoderm j according to my 

 own observations in the Medusas* (1864), according to the 

 investigations of Kollikerf " in Medusse and Hydroid Polypes 

 without exception " (1865), and according to the statements of 

 AllmanJ in the Sertulariai and Tubularige (1871). 



The still unpublished investigations of Dr. Gottlieb von 

 Koch also agree with this ; and he has shown me numerous 

 preparations of Coralla {Veretillum, Cereanthus, &c.) and of 

 Hydroids {Coryne, Tuhularia, &c.) which seem to prove 

 undoubtedly the origin of the ovicells from the epithelium of 

 the gastro-canal spaces. 



* "Die Familie der Riisselquallen {MeduscB Genjcmida),'''' Jenaisclie 

 Zeitschr. Bd. i. 1804, p. 449. 



i Icones Histologicaj, Heft ii. 1865, p. 89. 



X Monograph of the Gymuoblastic or Tubularian Hydroids, 1871, p. 149. 



