Though I will not now attempt to point out the full bearing of these facts on the study of embryology generally, yet I cannot resist calling attention to the similarity of the development of Magosphœra with the first stages of development of other animals, because it appears to me to possess a significance, the importance of which it would be difficult to overestimate.
Among the Zoophytes Prof. Allman thus describes[79] the process in Laomedea, as representing the Hydroids (Pl. [VI], Fig. 1, represents the young egg):—"The first step observable in the segmentation-process is the cleavage of the yolk into two segments (Pl. [VI], Fig. 2), immediately followed by the cleavage of these into other two, so that the vitellus is now composed of four cleavage spheres (Pl. [VI], Fig. 3)." These spheres again divide (Pl. [VI], Fig. 4) and subdivide, thus at length forming minute cells, of which the body of the embryo is built up.
In Pl. [VI], Figs. 5-9 represent the corresponding stages in the development of a small parasitic worm—the Filaria mustelarum—as given by Van Beneden.[80] The first process is that within the egg, which represents, so to say, the encysted condition of Magosphœra, the yolk divides itself into two balls (Pl. [VI], Fig. 6), then into four, eight, and so on, the cells thus constituted finally forming the young worm. I have myself observed the same stages in the eggs of the very remarkable and abnormal Sphærularia bombi.[81]
Among the Echinoderms M. Derbès thus describes the first stages (Pl. [VI], Figs. 10-13) in the development of the egg of an Echinus (Echinus esculentus):—"Le jaune commence à se segmenter, d’abord en deux, puis en quatre et ainsi de suite, chacune des nouvelles cellules se partageant à son tour en deux."[82] Sars has observed the same thing in the star-fish.[83]
PLATE. 6.
In the Rotatoria, as shown by Huxley in Lacinularia,[84] and by Williamson in Melicerta,[85] the yolk is at first a single globular mass, the first changes which take place in it being as follows:—"The central nucleus becomes drawn out and subdivides into two, this division being followed by a corresponding segmentation of the yolk. The same process is repeated again and again, until at length the entire yolk is converted into a mass of minute cells." Among the Crustacea the total segmentation of the yolk occurs among the Copepoda, Rhizocephala, and Cirripedia. Sars has described the same process in one of the nudibranchiate mollusca[86] (Tritonia), Müller in Entochocha,[87] Haeckel in Ascidia,[88] Lacaze Duthiers in Dentalium.[89] Figures 18 to 21, Pl. [VI], are taken from Koren and Danielssen’s[90] memoir on the development of Purpura lapillus.
Figs. [22-24] show the same stages in a fish (Amphioxus) as given by Haeckel, and it is unnecessary to point out the great similarity.
Lastly, figures 25 to 29, Pl. 6, are given by Dr. Allen Thomson,[91] as illustrating the first stages in the development of the vertebrata.