made at Naples in the icinter j)f 1871-72. 83 



continual division at the free edge of the cap which they form ; 

 and tkei/ groio over, the large nuclei.^ which are thus seen to lie 

 in a lower stratum of yelk than that along which the cap of 

 cells from the primitive segmentation is spreading. Both the 

 segmentation-cells and the large nuclei finally cover-in the 

 whole surface of the o.^^ as two distinct layers. This process 

 I observed over and over again, and repeatedly observed that 

 the large nuclei arise, each one separately, hy segregation from 

 the yelk-mass. They are products of free-cell formation, and 

 as such of the very highest interest in relation to histogenetic 

 doctrine. The cells which pass into the ovarian ^^^ in such 

 enormous numbers to swell its vitelline mass become so much 

 altered and broken down that it is not possible to regard these 

 large nuclei as descendants from them, though no doubt they, 

 as well as the original egg-cell's protoplasm and the fertilizing 

 male element, have contributed to form an organic "mixture," 

 if one may use the termj from which these free nuclei, as well 

 as the polar segmentation-disk and nucleus, take origin. 



The large pellucid nuclei subsequently become branched and 

 stellate ; whether they give rise to the whole of the elements 

 of the layer immediately below the outer segmentation-layer it 

 is not possible to say. If they do, contractile muscular cells 

 must be regarded as one of their products. In any case they 

 form the great bulk of the subepidermal tissue, that which cor- 

 responds with the mesoderm of vertebrates. 



Organs now begin to appear as thickenings on the sur- 

 face of the blastoderm : two eyes (the details of the pi-imitive 

 development of which are remarkable), two ears (which I 

 repeatedly saw in their primitive state as two pits, holes, or 

 in-pushings of the surface ; subsequently they present the con- 

 dition of capsules, each with a narrow canal opening on the 

 surface of the head, which canal becomes eventually the little 

 ciliated cgecum seen by Kolliker), and a median semicircular 

 primitive mouth ; besides these the mantle and arms — the 

 position of the latter marking off the blastodermic sac into two 

 parts, a great yelk-bag, and a smaller embryonic sac, which 

 gradually becomes more and more distinctly pinched off and 

 shaped out. It is not possible to say much of the further de- 

 velopment without illustration ; but I must mention two very 

 important facts. The primitive semicircular mouth is not the 

 real mouth. It is at this point that an inward cellular growth 

 commences, which eats its way into the mass of homogeneous 

 yelk lying in the embryonic portion of the blastodermic sac, 

 and meets (how or where exactly my observations do not show) 

 a shorter ingrowth from the anal aperture, in connexion with 

 which is also developed the ink-bag, thus agreeing with the 



6* 



