230 PROFESSOR HUXLEY ON THE ANATOMY 



embryo proper, as the foetus, reserving the term embryo for the blastoderm and the 

 results of its modification. In such a foetus as that represented in fig. 10, the blastoderm 

 is a broad, elongated, membranous patch g^rd of an inch long by x^th of an inch wide, 

 and so opake as at once to strike the eye when the foetus is viewed with even a very low 

 power. It is composed of somewhat coarse, granular-looking corpuscles, and lies between 

 the membrana propria and the modified epithelium ; but the former is separated from it 

 by a very thin layer of structureless siibstance which extends for some little distance 

 beyond the limits of the blastoderm on each side. The further course of development 

 shows that this layer is the rudiment of the test of the future ascidiarium. 



Foetuses of very slightly increased or even of less size exhibit a marked change in the 

 embryo, which has elongated sufficiently to extend over half the circumference of the ovi- 

 sac and has, at the same time, become indented at opposite points of its margins, so as to 

 be marked out into five short segments. One of the two terminal segments becomes much 

 enlarged, spreading over and investing one pole of the ovisac like a cujd ; while the other 

 four remain far smaller, and, the indentations between them deepening, they are 

 eventually connected only by narrow isthmuses of blastoderm. These segments are the 

 rudiments of as many zooids ; but the large cup-like one has a totally different fate from 

 the rest, and for distinction's sake I shall term it the cyathozooid, while the others are, in 

 their order of nearness to it, the 1st, 2nd, 3rd, and 4th ascidiozooids * respectively. The 

 zooids are not merely connected with one another by the isthmuses of blastoderm above- 

 mentioned, but the structureless test has greatly increased in thickness, and now invests 

 them all, like a thick layer of transparent varnish. The membrana propria of the ovisac 

 is no longer distinguishable outside this rudimentary test. 



The remains of the duct are often still traceable, towards the conclusion of this stage, 

 at one end of an equatorial diameter of the foetus (supposing the cyathozooid to be situated 

 at one of its poles) ; but later, it is no longer to be discovered. 



Eighth Stage. Foetuses from -^§tli of an inch tip to the largest loldcli have been met loith. 



In describing tliis final stage of development, it will be convenient to consider, first, 

 the changes in general arrangement, size and form, of the different parts of the foetus ; 

 and secondly, the special modifications which each of these parts undergoes. 



The cyathozooid, at fu-st, occupies but a comparatively small segment of the surface of 

 the spheroidal foetus, and the slightly curved series of ascidiozooids stretches out from it, 

 over about half the circumference of the uncovered portion of the ovisac (PL XXXI. fig. 

 11). But, by degrees, the cyathozooid extends so far as to invest nearly half the surface 

 of the ovisac, and, at the same time, the chain of ascidiozooids (considered as a whole) 

 gradually assumes a new direction, and applies itself closely to the face of the cyathozooid, 

 whose circumference it half encircles (fig. 13). Tlie blastoderm of the ascidiozooids, 

 however, remains perfectly distinct from that of the cyathozooid, the two being united 

 only by the layer of test, which, in the earUer stages, invested both, and whose con- 

 tiguous edges now seem to run into one another. 



*I have, throughout the present memoir, used the term ' ascidiozooid,' as more euphonious than 'ascidiite,' 

 employed in my notice in the ' Annals of Natural History' for 18G0. 



