PRELIMINARY ACCOUNT OF DEVELOPMENT OF LAMPREYS, 147 
vesicle. In short, the maturation of the egg, it seems, offers no 
such great peculiarities as Calberla supposed. 
Segmentation.—According to my results this process takes 
place as Max Schultze! has described it, 7. e. as in the case of 
the frog, or rather that of the newt, and not, as Calberla supposed, 
in such a way that epiblast and hypoblast are distinguished by the 
first division. At the end of segmentation the ovum is very 
similar to that of the triton, or sturgeon, of a correspond- 
ing stage. There are two kinds of blastomeres, the larger form 
the lower half of the egg, the smaller ones the upper half. The 
quantity of food-yolk is less than in the eggs just mentioned ; 
the segmentation cavity is extraordinarily large, and lies almost 
entirely in the upper half of the egg; its cover is made up of 
several layers of cells, of which only the exterior will develop 
into epiblast. The presence of cells in the roof of the segmenta- 
tion cavity, which will eventually belong to hypoblast, occurs 
otherwise only in such eggs as have a very large quantity of food- 
yolk, e.g. that of Accwpenser.? In general, epiblast and hypo- 
blast are roughly distinct at the end of segmentation, but the 
strict differentiation, as well as the foundation of the mesoblast, 
is produced by the well-known process of invagination. This 
is preceded by the thinning out of the roof of the segmentation 
cavity, which now consists, for the greater part of its extent, of 
a single layer of cells. The invagination is precisely similar to 
that of the newt.? By this means are formed in the median dorsal 
line two layers of cells, the epi- and hypo-blast, whilst on the 
sides we find these two and a third, the mesoblast. In the head 
and anterior part of the body the germinal layers are formed only 
in this way, while in the greater part of the body the ventral 
part of the mesoblast, and by far the greater portion of the 
hypoblast, arise by differentiation of the yolk-cells. 
This description of the formation of the germinal layers is 
very different to that which Calberla* gives. The discussion of 
his views would, however, lead us too far, and so it must be 
deferred. 
I can completely confirm Calberla’s results as to the formation 
of the notochord. It is formed from the invaginated hypoblast 
alone, but when it becomes detached from this layer it grows 
considerably further forward in the head than the hypoblast of 
the alimentary canal extends. 
1M. Schultze, ‘Die Entwicklung von Petromyzon Planeri,’ Haarlem, 1856. 
2 Salensky, ‘Development of the Sturgeon’ (Russian), Part 1, Kasan, 1879. 
3 Scott and Osborn, “On Some Points in the Early Development of the 
Newt,” this Journal, 1879. 
4 Calberla, “Zur Entwicklung des Medullarohres und der Chorda dor- 
salis, ete.,”” ‘Morph. Jahrbuch,’ Bd. iu. 
