174 whitman: [Vol.1. 



We have seen that radial symmetry, so far as outward appear- 

 ances go, is preserved in the &^^ of Clepsine until the eight-cell 

 stage is reached ; and that bilateral symmetry attains its full- 

 est expression through the cleavage of the posterior macromere, 

 which ends in the establishment of ten teloblasts. In Eupoma- 

 tus we reach the same important stage of development by the time 

 the blastopore has been reduced, by closure advancing from be- 

 hind forward, to a small pore, — the future mouth. The radial 

 Gastrula has passed into the bilateral, embryonic stage of the 

 larva. The teloblasts are represented by a pair of mesoblasts 

 (" pole-cells " ) and a pair of nephroblasts. The neuroblasts are 

 not distinguishable, which may be explained on the supposition 

 that they still lie in the ectoderm, and present no conspicuous 

 differential characters. A prae-oral band of cilia (prototroch, 

 Kleinenberg), occupying an equatorial position with respect to 

 the primary axis, is already present, and the apex of the prae- 

 oral lobe (Scheitelfeld) bears a few long cilia. With the ex- 

 ception of the ring and the tuft of cilia, the organs of the larva 

 are as yet undeveloped, and exist only in the form of more or 

 less definite rudiments. The embryonic TrochospJiere, as we may 

 call this stage, is represented in Hatschek's Figs. 25 and 26, 

 PI. XI. 



In Clepsine, as I have said, the formation of the teloblasts, 

 as the closing act of cleavage, brings us to the stage which cor- 

 responds most nearly to the embryonic Trochosphere. For the 

 sake of distinction this stage (Diag. 4) may be named the y^/^/ 

 Trochosphere. 



In both forms we meet with the same fundamental features, 

 and the differences are precisely such as general principles 

 would lead us to anticipate. Ciliated organs of locomotion, 

 specially adapted to the needs of a roving, larval life, but with- 

 out functional importance for the foetus, are not developed in 

 Clepsine. As the foetal Trochosphere is supplied with a large 

 stock of food-material, ready for absorption without the aid of a 

 digestive system, there is no necessity for an early development 

 of the mouth and gastric cavity, such as must exist in the case 

 of the embryonic Trochosphere; and, accordingly, we find the 

 larval development far in advance of the foetal in these particu- 

 lars. In respect to the trunk-bud (teloblasts), the case is re- 

 versed; for in the larval embryo the differentiation of the bud 



