Prof. T. H. Huxley on the Development of Pyrosoma. 33 



blastoderm is a very remarkable change in the epithelium of 

 the ovisac, which becomes converted into a thick transparent 

 substance containing many spheroidal excavations, which are, 

 apparently, the cavities of the primitive epithelial cells. 



I have little doubt that the germinal spot divides and sub- 

 divides, and that each of its portions appropriates to itself a 

 segment of the semisolid contents of tlie germinal vesicle, to 

 form the cndoplast and its investment or one of the constituents 

 of the blastoderm. At present, however, I have no absolute 

 proof that such is the case, though I have seen a germinal 

 vesicle with four or five granules of unequal size in the place of 

 the germinal spot. 



The blastoderm rapidly enlarges, and forms an oblong patch, 

 which extends over a gradually increasing segment of the 

 circumference of the ovisac, between the membrana propria 

 (which gradually disappears over it as a distinct structure) and 

 the epithelium, and at the same time becomes covered, with a 

 thickish layer of transparent substance, the rudiment of the 

 future cellulose test of the Pyi^osoma. 



The elongated band-like blastoderm next becomes marked 

 out, by four constrictions, into five portions. That segment 

 which terminates one end of the series enlarges faster than the 

 others, and invests one end of the ovisac with a sort of cap ; the 

 other four portions assume a heart-shape, the base of each 

 * heart^ being turned towards the ' cap.' AH five portions remain 

 connected by narrow necks. In this stage the blastoderm ex- 

 tends over one half of the circumference of the ovisac, which is 

 about 3^\jth of an inch in diameter. 



Up to this point the ovisac has remained within a chamber of 

 the sinus-system of the parent zooid, by whose blood it is bathed 

 on all sides ; but it has gradually thrust the atrial wall of that 

 chamber before it, and eventually it breaks through the wall, 

 and passes into tlie atrial cavity, a very large portion of which it 

 occupies. The duct of the ovisac (now a mere thin cord as com- 

 pared with the whole ovisac) is broken through in the course of 

 this curious natural Caesarian parturition, and soon ceases to be 

 discoverable. 



The young blastoderm itself is, as I have said, almost directly 

 bathed by the parental blood so long as the ovisac remains in 

 the sinus, while the thin walls of tlie ovisac must allow of such 

 an abundant ])assage of nutritive matter into its interior that it 

 must serve as a great reservoir of material for the develo])ing 

 embryo after birth. The contents of the ovisac, therefore, though 

 neither a food-yelk nor a placenta, serve the purpose of both. 



It would b(; impossible, without the figures with which I pro- 

 pose to illustrate the memoir presented to the Linnean Society, 



Ann.l^ M(i(j.N.Jlist. Ser. 3. Vul.w 3 



