TROPIOMETRA CARINATA. 9 



In some cases I have found, instead of an embryo, the whole space 

 inside the egg-membrane filled by a uniform mass of minute spherules, 

 besides a couple of deeply staining protoplasm masses, looking like a pair 

 of cleavage cells, but containing no nuclei. This would appear to be some 

 kind of parasitic organism. It has seemed to me worth while calling 

 attention to this, although I am unable to say more definitely what it is 

 (plate II, figiu-e 8). 



When the gastrula stage is reached, the embr3'^o begins to rotate within 

 the egg-membrane, being covered with a uniform ciliation. About 6 hours 

 after the fertilization the rupture of the egg-niemhrane takes place and the embryo 

 swims out. Only a smaU opening is formed in the membrane, through which 

 the embryo must squeeze itself out. The empty membrane may be found 

 on the bottom of the jar, undisturbed, except for the hole through which the 

 embryo has crept out. 



The embryo, just after the liberation, is shghtly pear-shaped, being a 

 little pointed at the apical end and a little truncated at the oral end. There 

 is thus no difficulty in seeing directly that the place of the blastopore is the 

 posterior end of the larva; further, this shape of the embryo facilitates the 

 orientation in sectioning, the longitudinal axis being alwaj^s distinct from the 

 moment the embryo is hberated, while the spherical embrj'o of Antedon 

 can not be oriented with certainty in sectioning until the vibratile bands 

 have been formed, the situation of the archenteron nearer the posterior end 

 and the more numerous mesenchyme cells in the anterior end affording the 

 only means by which to identify the longitudinal axis (Seehger, pages 

 171, 200). 



The ciliation is still quite uniform, and there is no indication as yet of 

 the ciliated bands. The blastopore closes ivwiediatelij after the liberation. 

 Plate II, figiu-e 4, represents a longitudinal section through an embrj-o only 

 6 hours old, immediately after the liberation. It shows the blastopore 

 closed, and the formation of the mesoblast cells from the upper end of the 

 archenteron has already made fair progress. 



2. FORMATION OF THE CCELOMIC VESICLE. SIXTH TO TWELFTH HOURS. 



At this stage the ciliation is still uniform; in embrj'os 10 hours old the 

 apical tuft of ciha is distinct, but the cihated bands do not begin to appear 

 until the embryo is 12 hours old. 



The ectoderm, which in embryos 6 hours old (plate ii, figure 4) is a typical 

 epithehum, with the nuclei arranged fairly regularly in a single series at the 

 basal end of the cells, is considerably thickened with the nuclei arranged 

 pluriserially (plate ii, figure 5; plate in, figures 1 to 4); it is, however, still 

 distinctly a single-laj'ered epithelium. At the anterior end under the apical 

 tuft it is more or less thickened (plate iii, figures 3 and 4) ; this part corre- 

 sponds to the apical pit of Antedon, but it is never so conspicuous as in that 



