28 MR. E. RAY LANKESTER ON THE 
No. HI. The Early Development of Tergipes, Polycera, Tethys, Neritina, Limax, and 
Limneus. 
Tergipes.—The early history of the development of some of the Nudibranchs is of 
considerable interest, since it clearly exhibits a Gastrula phase similar to that described 
in my contribution on Pisidium, but which I could not discover in Aplysia. CARL 
Voer, in his memoir on the development of Actwon, has described and figured the 
“ sillon”” which results from the invagination of the wall of the primitive blastosphere ; 
but he did not distinctly recognize it as an invagination, nor are his figures sufficiently 
large to give much information on the subject. 
In Plate 9 the early development of a small Tergipes, the species of which I did not 
identify, but which was common at Naples, is given. The invagination is very well 
marked in this case, since there is relatively but a small amount of “ deutoplasm ” 
present in the egg, that constant disturber of typical modes of development and of 
satisfactory observation of the eggs by transmitted light. 
Plate 9. fig. 1. The cleavage-cells do not present great disproportion in size. 
Plate 9. fig. 2. Already in the centre there is a pit due to the tucking in of the 
cleavage-products. 
Plate 9. fig. 3 gives a later embryo in optical section. The invaginated group of 
cells (hy) are seen lying within the wall-forming cells (ep). The cavity of the invaginated 
group (C) still communicates with the exterior. 
Plate 9. fig. 4 is an optical section at right angles to the preceding, so that the aper- 
ture of invagination is not brought into view. 
Plate 9. fig. 5. A surface-view of an embryo at the same stage, showing the long 
groove formed by the aperture of invagination. This is the groove detected by Voer 
in Actwon. It closes up shortly, and the layers of the embryo proceed on their special 
lines of development. 
Plate 9. fig. 6 shows the embryo with the aperture of invagination or Gastrula- 
mouth* now closed. The velar circlet of cilia has developed, and the two layers of the 
embryo are breaking up into smaller and specially differentiated cells or corpuscles. 
Plate 9. figs. 7, 8 show the separation of a middle layer (me) between the inner 
and outer. It appears to be derived from the epiblast, to judge from the appearances 
seen in Plate 9. fig. 7; but the hypoblast may also contribute to its formation. 
Plate 9. fig. 9. Surface-view of the aboral pole of an embryo of the same stage as 
the preceding, showing a fold or scar which is the remnant of the primitive invagination 
aperture. 
Plate 9. fig. 10 represents an embryo in which velum (v), foot (f'), and shell (sh) 
are already taking form. The pharyngeal invagination (0) is also indicated. 
In this and preceding figures two small cells are marked R, which appear to be 
“ Richtungsblischen ;” they are well known in the development of Mollusca. 
Plate 9. fig. 11 represents the embryo at a much more advanced stage; the shell is 
* March 7th, 1875.—Better called “ blastopore,” since it is not known to represent a mouth. 
