[102] Rabl (No. [268]) has quite recently given a more detailed account than previous observers of the origin of the mesoblast in Planorbis. He finds that it originates from the posterior one of the four large cells which remain distinct throughout the segmentation. By the division of this cell two ‘mesoblasts’ are formed, one on each side of the middle line at the hinder end of the embryo. Each of these again divides into two, an anterior and a posterior. By the division of the mesoblasts there arise two linear rows of mesoblastic cells—the mesoblastic bands—which are directed forwards and divided transversely into two parts, an anterior continued from the front mesoblast, and a posterior from the hinder mesoblast.
If Rabl’s account is correct, there is a striking similarity between the origin of the mesoblast in Mollusca and in Chætopoda. It appears to me very probable that the mesoblastic bands are formed (as in Lumbricus) not only from the products of the division of the mesoblasts, but also from cells budded off from one or both of the primary germinal layers.
[103] In Octopus and Argonauta (Lankester) as soon as the blastoderm is completed the egg reverses its position in the egg-shell; the cleavage pole taking up a position nearest the stalk.
[104] I do not know the relation of this axis of symmetry to the future embryo.
[105] “Development of Pond-Snail.” Quart. J. of Micro. Science, 1874, pp. 371‑374.
[106] There is a striking similarity between the changes of the blastopore in Chiton and the formation of the neurenteric canal of Chordata; especially if Kowalevsky is correct in stating that the pedal nerves are developed from the ventral plate.
[107] R. H. Peck, “Gills of Lamellibranch Mollusca.” Quart. J. of M. Science, Vol. XVII. 1877.
[108] The account of the remainder of the development till the larva becomes hatched is taken from Rabl, No. 292.
[109] In this description I follow Rabl’s nomenclature. According to his statements the ventral part of the body is the original animal pole—the dorsal the lower pole; the anterior end the mesoblastic side of the opening of invagination.
[110] The position of the foot and gills in the larva represented in [Fig. 119] B would be more normal if the convex and not the flatter side of the shell were the anterior. I have followed Rabl and Flemming in the determinations of the anterior and posterior end of the embryo, but failed to rear my larvæ up to a stage at which the presence of the heart or some other organ would definitely confirm their interpretation. I originally adopted myself the other view, and in case they are mistaken, the so-called velum would be a circumanal patch of cilia, while the position of the primitive mesoblast cells as well as of the byssus would better suit my view than that adopted in the text on the authority of the above observers.