M. Ussow's Zoohfjico-Emhryological Investigations. 211 



sections appears as a shallow but wide depression of the upper 

 germ-lamella. At the time of its appearance the rudiment of 

 the mantle is situated in the middle of the original germinal 

 disk {cent)-um), with by far the greater part of it on the dorsal 

 surface, whilst the somewhat elevated (constricting) part which 

 subsequently grows round the ventral surface occupies only a 

 very inconsiderable space upon the latter. 



Above the mantle there are symmetrically on the two sides 

 of the dorsal surface the two eye-ovals, and between them, at 

 the boundary of the region of the arms, the above-mentioned 

 rudiment of the buccal orifice. The lateral surfaces of the 

 embryo represent the future cej)halic lobes. 



On the following day, in all the Cephalopoda investigated by 

 me, the branchicp, the funnel^ the anns^ and the anal tubercle 

 made their appearance. At the time when the rudiment of 

 the mantle has become rather more constricted oft" from the 

 blastoderm on the ventral surface, the cell-layer of the upper 

 germ-lamella becomes somewhat thicker at the sides of the 

 embryo (at first by longitudinal division, by which the cells 

 are rendered higher, and then also by transverse division), 

 and forms two inconsiderable prominences, which gradually 

 grow and are the rudiments of the two so-called cejjhalic 

 lobes. 



As regards the rudiments of the branchiag, which are at first 

 situated on the ventral side of the embryo not far from the 

 margin of the mantle, these are developed from the more than 

 one-layered thickening * of the dermo-muscular layer of the 

 middle germ-lamella, Avhich is covered by the cells of the 

 upper lamella. 



On the boundary between the anterior cephalic lobe and the 

 rudiment of the mantle a semilunar fold makes its appear- 

 ance on each side of the embryo, produced by a thickening 

 of the dermo-muscular layer, and covered, like all the organs 

 mentioned, by cells of the upper lamella. This is the rudi- 

 ment of the fannelj which consists of two halves, the margins 

 of which coalesce very late, iiuleed only at the commencement 

 of the third period f. 



Almost simultaneously with the appearance of the branchiae 

 there is formed between their pyriform rudiments, in the 



* At the end of the second and during the third period the cells in the 

 middle of the solid branchial rudiments <rradually become loosened, and 

 tortuous reticulated ducts are produced in which the branchial arteries 

 and veins with their numerous ramitication.-j are formed (see Van Beneden, 

 /. 0. p. 9 ; Kollilver, /. c. p. 89 ; Metschnikoff, /. c. p. 61). 



t On tlie dorsal surface tlie two halves of the funnel approach each 

 other as early as the fourth day of the second period. 



