DEVELOPMENTAL HISTORY OF THE MOLLUSCA. 8 
to call the Planula, but which Professor Harcket has better termed the Gastrula, 
reserving the former name for a condition of the Gastrula which sometimes presents 
itself in which there is no aperture of invagination. The Gastrula thus formed consists 
of an outer and an inner layer of cells, forming a wall which encloses a cavity (the 
primitive gastric cavity) which communicates with the exterior by the aperture of 
invagination. The two layers of cells are thus respectively the representatives of the 
ectoderm and endoderm of Ceelenterata, and further of the epiblast and the hypoblast 
of the developing Vertebrate. 
Up to this point the membranous envelope of the egg is intact ; but now it disappears, 
since the egg increases very largely in size (Plate 1. figs. 9, 10, 11). This increase in 
size is due to the rapid growth of the outer layer of cells, which expands and separates 
itself entirely from contact with the invaginated layer, excepting at the part corresponding 
to the lips of the orifice of invagination. The orifice entirely closes, and the primitive 
gastric cavity remains as a small shut sac formed by well-marked cellular elements, 
affixed to one part of the large expanding epiblast, ectoderm, or outer cell-layer (see 
fig. 10). ‘The space between the two primitive layers is occupied by a colourless trans- 
parent liquid. A surface-view of an embryo in this stage is given in fig. 9, figs. 10 & 11 
being deeper views of the same embryo. 
The surface-cells are seen to lie closely packed, with a small quantity of granular 
matter surrounding each large clear nucleus. ‘The granular matter represents the body 
of each cell, and is apparently in this condition not distinctly demarcated for each indi- 
vidual element; so that the epiblast is in the condition of a granular protoplasm with 
numerous closely packed imbedded nuclei. ‘The clear pellucid nuclei present one, two, 
three, or four nucleoli, and are in process of multiplication by fission. 
When the focus of the microscope is so adjusted as to bring an optical section of the 
embryo into view, we get the appearance given in fig. 10; the invaginated hypoblast is 
seen as a small oblong mass (Ay) at one pole of the oval embryo, and the wall formed by 
the epiblast, which is only one cell thick, is seen in section. 
But now closer examination shows here and there fusiform or branched cells (me) 
attached to the inner surface of the epiblastic wall. More careful focusing, so as 
to bring this surface precisely into view, gives the appearance represented in fig. 11, 
where a larger number of these subjacent branched cells are visible. 
These branched cells are the commencement of the mesoblast. The great space between 
the invaginated hypoblast and the epiblastic wall is the mesoblastic cavity, that cavity 
which is the distinguishing characteristic of the higher groups of the animal kingdom, 
and which becomes ultimately blood-sinus, peritoneal cavity, or hemolymph-system. 
The minutest details as to the mode of origin of these first mesoblastic cells would be 
of the greatest interest in the present state of our knowledge as to the origin of the 
middle layer of the Vertebrate embryo, and I accordingly have paid especial attention 
to it. 
Plate 1. figs. 12 & 13 give more highly magnified views of parts of the embryo 
B2 
