534 
PROFESSOR W. THOMSON ON THE EMBRY 0 GEN Y OE ANTEDON 
The perfect organic and relative life of this being, closely comparable to the life of 
the most highly gifted members of the protozoic snbkingdom, does not certainly exist 
in the sum of the permanent organs ; it resides, I believe, simply in a pseudembryonic 
sarcodic layer, endowed with the same properties which this zoological element possesses 
when isolated, as in the Protozoa. Gradually the sarcode eliminates from the products of 
its own assimilation the constituents, and elaborates the tissues, of the permanent special 
organs; and when these are sufficiently developed, it loses its own individuality, its 
vital activity passing into the organs which it has produced, and performing through 
their medium more effectively and condensedly, functions, which, as a transient nurse- 
layer, it performed in a manner perfect as to its simple object of temporary nutrition, 
though somewhat feeble and diffuse. In respect to the essentials of this process, some 
of the Holothuridea among the Echinodermata seem to conform almost exactly to the 
ordinary Invertebrate type. The pseudembryonic sarcode-layer is here little more special 
or independent than it is in the embryos of the Annelids and Mollusks, and infinitely 
less so than in some Turbellarians ; and the transition from this condition, through the 
Crinoids, in which a short alimentary canal is formed in the sarcode layer, — and the 
“Plutei” in which the “ Echinoderm disk” with its accompanying permanent organs is 
developed within the pseudembryo and covered by its general integument, the whole 
substance of the pseudembryo being finally absorbed into the embryo, — to the “ Bipin- 
naria,” in which the independent life of the pseudembryonic zooid is apparently carried 
to its limit, is so perfectly gradual as to leave no doubt whatever of the uniformity of 
the embryogenic plan. 
This being the case, that is to say, a vast number of invertebrate embryos combining 
in their earlier stages pseudembryonic appendages possessing independent vitality with 
the nascent organs, no special divergence from the ordinary mode of development is to 
be anticipated in cases in which the pseudembryo attains unusual individual indepen- 
dence. We find accordingly the earlier stages in the development of the pseudembryo 
in the Echinoderms conforming closely to the general mode of development of the 
“ embryo” of aquatic invertebrates. 
The earlier stages in the development of the Tissues of the Pentacrinoid. 
The general connective tissue. — As stated above, the general transparent investment 
which during the earlier stages of its development makes up the greater portion of the 
substance of the pentacrinoid, is produced by the gradual extension and modification of 
the sarcode substance of the pseudembryo. The pseudembryo is moulded from the 
germ-mass, and at first its surface retains the mammillated structure, the result of the 
ultimate segmentation of the yelk. At first each spherule retains a trace of the original 
enclosed endoplast ; this, however, shortly disappears. No cell-membrane can be detected 
investing these spherules at any period. An hour or two after the rupture of the vitel- 
line sac, the mammillated structure entirely disappears, the ultimate spherules being 
fused into a structureless layer. The external layer is firm and consistent. If the 
