356 



INVERTEBEATA 



CHAP. 



peripheral blastomeres of the blastoderm, at right angles to the surface. 

 These divisions take place along what afterwards are seen to be 



the posterior and lateral edges 

 of the blastoderm. In this way 

 an incomplete ring of what are 

 called "lower layer cells" is 

 formed, which former observers 

 have termed mesoderm but which 

 we may term mesendoderm. 

 Lankester (1875) maintains that 

 there is a " primitive streak " 

 in the segmenting eggs of Cephalo- 

 poda, by which he means a re- 

 stricted area of the surface over 

 which the proliferation of cells 

 which gives rise to the mesendo- 

 derm takes place. A priori this 

 is very likely, although so far 

 this observation has not been 

 confirmed. This mesendoderm 

 grows inwards under the blasto- 

 derm to a considerable extent, 

 but does not invade, except in 

 the form of a few loose scattered 

 cells, its front edge or its centre. 



The formation of definite 

 organs in the embryo of Lolifjo 

 has been worked out in com- 

 paratively recent times by Korschelt (1892) and by Faussek (1900). 

 These workers found that Hermann's fluid, and similar mixtures 

 containing osmium tetroxide, render the tissues too brittle for section- 

 cutting although giving excellent histological detail. They therefore 

 employed picro- sulphuric acid and Perenyi's fluid, which gave 

 excellent preservation of form, although the histological detail is 

 not so perfect as that obtained by the use of Hermann's fluid. 



The first organ to make its appearance is the shell gland. This 

 appears as a heart-shaped area of columnar ectoderm in the mid- 

 dorsal line. This thickening in fact occupies the apex of the egg ; in 

 front of it is what we may call the anterior slope, behind it the 

 posterior slope. Soon an invagination appears in the centre of this 

 thickening ; but, as Lankester long ago pointed out (1875), this does 

 not correspond to the invagination of the shell gland which we 

 encounter in other Mollusean embryos. That invagination precedes 

 the condition of the gland when it is a saddle-shaped area, and 

 lasts only a short time, but the invagination in the shell gland area 

 of a Cephalopod lasts throughout Kfe; it constitutes, in fact, the 

 shell-sac which encloses the shell (a horny pen in Loligo, but a 

 complex calcareous structure in Sepia). From a comparison with 



Fig. 286. — A portion of the margin of the 

 blastoderm of the egg of Sepia officinalis 

 at the conclusion of the process of seg- 

 mentation ; to show the transformation 

 of the blastocones into cells of the yolk- 

 menibrane. (After Koeppern, from 

 Minchin's preparations. ) 

 &ic, blastocones ; y.m.c, yolk-membrane cells. 



