Preliminary Note on the Mesoderm Formation 



of Pulmonata. 



T. Fujita. 



(With Plate XIV.) 



Our knowledge of the mesoderm formation in gastropods has been 

 recently formulated by F. Schmidt * thus : " Eine Uebereinstimmung 

 in del' Bildung des iNIesoderm deu Gastei-opoden scheint also wenigstens 

 dariii constatirt odei* doch sehr wahrscheintlich gemacht zu sein, dass 

 es seinen Ursprimg aus dem piimuren Entsderm ninimt, sei es nun 

 sclion fruhzeitig in Gestalt zweier oder eiiiiger vvenigen Urinesoderm- 

 zellen odeu erst bedeutend spiiter in Form einer Ausstiilpang aus dem 

 Urdarm." It requires no comment that more observations on the point 

 are exceedingly needful. INIy embryological studies on Siplionaria lepida 

 (lid., carried on since the spring of last year, led me to certain results 

 on the early stages, which shall here ])e briefly noticed reserving 

 full descriptions and discussions to a fature occasion. 



The laid egg of Siphonar'ni lepida are approximately spherical in 

 shape with a diametsr of 0.1 mm. Each egg is contained somewhat 

 eccentrically in an ovoid chorion-sac, the space between the two being 

 filled with coarse refractive splierules (fig. 1, PI. XIV.}. So long as these 

 spherules persist, the early changes of the ovam can only be studied by 

 means of serial sections. Three or four hours after egg deposition, 

 the germinal vesicle gives place to a karj^okinetic figure that leads to 

 the expulsion of the first polar body. At about this time a spermato- 



* Fordin.and Sohmidt. Die Fiirelumo- uiid Koiin-Jilatterliildiuig der Stylouuiiato- 

 plioren. Zool. .Tahr).. l^d. VIT. 1891. 



