Notes on the Development of Sepia 107 



of the germ calotte, and more recent researches (e.g., Faussek) have corrob- 

 orated his account in main outline and only added a few details. The difficulty 

 arises when we come to view the yolk epithelium and the other products of 

 segmentation in the light of the germ-layer theory. Ussov, Bobretzky, Bruce, 

 and Vialleton are unanimous in terming the yolk epithelium the endoderm. 

 Subsequent investigations have made this purely topographical criterion of 

 nomenclature untenable, especially after it had been proved without doubt 

 (Faussek, Distaso) that the yolk epithelium does not only not participate in 

 the formation of the mid-gut, but also has nothing whatever to do with the 

 actual construction of the animal, beyond aiding it in its early metabolism. 

 Although it is justifiable to draw a comparison between the blastocones and 

 the macromeres in other telolecithal ova (cf., Teleosteans), I do not consider 

 it expedient to term a structure the endoderm when its cells do not possess 

 any histogenetic properties whatsoever, nor its chromatin the hereditary 

 qualities for any part of the adult. In some records (Faussek) a com- 

 promise has been made by calling the yolk epithelium the primitive 

 endoderm, and the layer between it and the ectoderm (ectoderm is the only 

 name which has been applied unambiguously by all) the mesendoderm. 

 The latter word, again, has been used by Korschelt to denote the Anlage, 

 situated at the under side of the periphery of the ectoderm, of both the yolk 

 epithelium (" endoderm ") and of the layer hitherto universally — with the 

 exception of Teichmann — accepted as " mesoderm " or " mesendoderm." 

 This medley of creeds may be obviated most effectively by dropping 

 "mesendoderm" here altogether, and either refraining from committing 

 oneself to any name for the layer from which inter alia the mid-gut originates 

 or substituting endoderm for the sometime " mesoderm " or " mesendoderm." 



Further research on the development of Sepia and its allies is urgently 

 required to ratify what is already known and elicit more details, and in 

 order to pave the way for a homogeneous embryology of the Cephalopoda. 



Literature. 



(1) Balfour, F. M. — A Treatise on Comparative Embryology. 2 vols. 



London, 1880. 



(2) Bobretzky, I. — Izliedovania o razvitii golobonogich Moscow, 1877. 



(3) Braem, W. — Was ist ein Keimblatt? Biol. Centr., xv., 1895. 



(4) Bruce, A. T. — Segmentation of the Egg and Formation of the Germ-Layers 



of the Squid (Loligo Pealii), Johns Hopkins Uidv. Circ, vi., 54, 1886. 



(5) Distaso, A.— Studii sull'embrione di Seppia, Zool. Jahrb., xxvi., 4, 1908. 



(6) Faussek, V. — Lntersuchungen iiber die Entwicklung der Cephalopoden, 



Mitt. a. d. Zool. Stat. Neapel, xiv., 1901. 



(7) Korschelt & Heider. — Text-Book of the Embryology of Invertebrates 



(English ed.), vol. iv. London, 1900. 



