610 W. J. SOLLAS. 



with a spherical highly refrin gent nucleolus (0*0015 mm. in dia- 

 meter) which stains deeply lies at the outer end or base of the 

 cell, where also the contained granules of the cell-protoplasm 

 appear to be most numerous. The opposite inner end of the 

 cell terminates in a faint but clearly defined rounded margin 

 against the structureless blastema, which fills the interior and 

 envelopes the exterior of the embryo. Owing to the position 

 of the nucleus the outer end of the cell is far more conspicuous 

 than the inner, and since the cells lie evenly side by side to 

 form the wall, it follows that this is much more darkly 

 bordered on its outer than its inner edge. 



Coincidently with the progressive folding of the walls of the 

 embryo the cells of both layers become modified. They mul- 

 tiply in number and diminish in size ; at the same time the 

 outer end becomes defined as a rounded head containing the 

 nucleus, and constricted from the pale faintly granular body 

 of the cell by a shallow groove (fig. 25). The cells continue 

 to grow smaller and more numerous till the approximation of 

 the two layers is complete (fig. 29), and perhaps subsequently. 

 At length, soon after the formation of the double wall, the 

 nuclei of the cells, now much reduced in size, retreat from the 

 outer end of the cell, where hitherto they have constantly 

 remained, towards the interior, and at the same time the 

 outer end becomes cleared of granules, so as to resemble the 

 collum of a choano-flagellate cell, and a flagellum is protruded. 

 No doubt, judging from a wide analogy, a collar is also pro- 

 duced, but search for this in spirit specimens is ineffectual, as 

 might be expected. The cells of both layers pass through 

 identical changes, and are as similar at the close of the deve- 

 lopment observed as at its commencement. 



On comparing the account here given of the embrylogy 

 of Halisarca lobularis with that observed by Metschnikoff, 

 Schulze, and Barrois, one is fairly staggered at its divergencies, 

 not to say contradictions. As to the fidelity of the observa- 

 tions of these investigators no one can suggest a doubt, and 

 as for mine the sections on which they are founded are pre- 

 served in balsam and open to inspection. Beyond the blastula 



