Fig. 199. Two stages in the development of Holothuria tubulosa, viewed in optical section. (After Selenka.)
A. Stage at the close of segmentation. B. Gastrula stage.
mr. micropyle; fl. chorion; s.c. segmentation cavity; bl. blastoderm; ep. epiblast; hy. hypoblast; ms. amœboid cells derived from hypoblast; a.e. archenteron.
There are various peculiar modifications of invagination which cannot be dealt with in detail.
Fig. 200. Transverse section through the ovum of Euaxes during an early stage of development, to shew the nature of epibolic invagination. (After Kowalevsky.)
ep. epiblast; ms. mesoblastic band; hy. hypoblast.
Invagination in one form or other occurs in some or all the members of the following groups:
The Dicyemidæ, Calcispongiæ (after the amphiblastula stage) and Silicispongiæ, Cœlenterata, Turbellaria, Nemertea, Rotifera, Mollusca, Polyzoa, Brachiopoda, Chætopoda, Discophora, Gephyrea, Chætognatha, Nematelminthes, Crustacea, Echinodermata, and Chordata.
The gastrula of the Crustacea is peculiar, as is also that of many of the Chordata (Reptilia, Aves, Mammalia), but there is every reason to suppose that the gastrulæ of these groups are simply modifications of the normal type.
2. Delamination. Three types of delamination may be distinguished:
a. Delamination where the cells of a solid morula become divided into a superficial epiblast, and a central solid mass in which the digestive cavity is subsequently hollowed out ([fig. 201]).