54 MR. E. RAY LANKESTER ON THE 
This and certain relations of bulk in the early-developed organs of the various embryos 
considered, determine the development either by invagination or by delamination. The 
relation of bulk to the process of invagination I may illustrate from a fact established 
in the preceding contributions. In Loligo* the large otocysts each develop by a well- 
marked invagination of the epiblast forming a deep pit, which becomes the cavity of 
the cyst. In Aplysia the smaller otocysts each develop by a simple vacuolation of the 
epiblast without invagination. Again, in Vertebrata the nerve-cord develops by a long 
invagination of the epiblast; in Huaves and Lumbricus the corresponding nerve-cord 
develops by a thickening of the epiblast without any groove and canal of invagination. 
The bulkier structures in these cases are seen to develop by invagination, the smaller 
by direct segregation. Invagination therefore acts as an economy of material, a hollow 
mass being produced instead of a solid mass of the same extent. 
A. Gastrule developed by invagination, or invaginate Gastrule, with either (1) embolé 
or (2) epibolé.—That the presence of a quantity of deutoplasmic matter, or of a partially 
assimilated mass of such matter, in the original egg is not accompanied by well-marked 
invagination of the blastosphere, whilst the absence of much deutoplasm is the invariable 
characteristic of eggs which develop a Gastrula by invagination, is shown by a com- 
parison of Aplysia and Loligo with Pisidium and Limax, and of the Bird with the 
Batrachian. In some cases, such as SELENKA has characterized by the term “ epibolé,” it 
seems that the enclosure of the large yelk-mass by the overgrowth of cleavage-cells may 
be held as an equivalent to the invagination of the large yelk-cells by “‘embolé;” and 
the intermediate character which the development of Ewaves and of Lumbricus presents 
in this respect, as described by Kowatnysky, tends very strongly to establish a transition. 
B. Gastrule developed by segregation, or segregate Gastrule.—But the mode of 
development of the Gastrula of Geryonide, described with so much minuteness by Fou, 
which is obviously the same as that of the Gastrulew of Spongiadz and most Hydroids, 
is clearly no masked case of invagination. There is no question of “ epibolé” here, but 
a direct and simple splitting of one cell into two; so that what was a sac formed by a 
layer of cells one deep, becomes a sac formed by a layer of cells two deep, or of two 
layers each one deep. It is yet a question for much further inquiry as to how this 
mode of forming a double-walled Gastrula can be derived from, or harmonized with, 
the formation of Gastrule by the embolic or epibolic forms of invagination. 
It would certainly seem, at present, that the orifice of invagination of the invaginate 
Gastrula must not be regarded as the equivalent of the later erupting mouth of the 
segregate Gastrula}, which is the true permanent mouth of the Sponge or Ceelenterate. 
* See Annals & Mag. Nat. History, Feb. 1873; also Proc, Roy. Soc. no, 151, 1874, and Quart. Journal of 
Microse. Sci. January 1875. 
+ In my paper in the ‘ Annals’ for May 1873, I have inclined to the view that it may be so regarded. 
In a paper written a year after the date of the present memoir, and published in the Quart. Journ. Mier. 
Science, April 1875, I have proposed to retain the original term Planula instead of Gastrula, and to speak of 
the orifice of invagination as the “ blastopore,”’ 
