SALPIDAE FORMS WITHOUT COVERING FOLDS. 423 



According to TODARO, the cells derived from the follicle-epithelium 

 which are fated to disintegrate, can still be distinguished between 

 these embryonic cells. It should, indeed, be mentioned that 

 SALENSKY was able to find within the embryo, long after cleavage 

 had ended, at a time when the first rudiments of organs grow dis- 

 tinct, a few large blastomeres definitely arranged. The significance of 

 these is obscure, and we are altogether in the dark as to those stages 

 in the development of Salpa which lie between cleavage and the be- 

 ginning of the formation of the organs, i.e., the stages in which we 

 should expect the germ-layers to form.* The cleavage-cavity appears 

 to be wanting in all Salpidae [SALENSKY, HEIDER and KOROTNEFF] . 

 BROOKS, however, suggests that the follicular cavity may be thus 

 interpreted. 



Certain divergencies are found in the different species of Saipa in 

 the further processes of development, but these apparently are not 

 due to any fundamental difference. The development of most species 

 being as yet only very partially known, we shall restrict ourselves to 

 a closer account of the two forms which have been best investigated, 

 viz., S. democrat ica-mucronata and /S. pinnata. These represent two 

 types of development which are to be distinguished by the absence 

 or presence of a covering fold and by the structure of the placenta. f 



A. Forms without Covering Folds. 



The embryo of Salfxi (Thalia) deniocratica-mucronata, in the stages 

 that mark the completion of the cleavage-processes (Fig. 211 C) pro- 

 jects in the form of a cone into the atrial cavity of the parent. Tt 

 lengthens later and becomes more cylindrical, its end being rounded 

 (Fig. 2125). It is still surrounded by the two walls of the brood- 



* [Recent investigations tend to show that germ-layers, as we understand 

 them in other animals, cannot be said to exist in Salpa, since the various 

 germ-cells, which give rise to the different organs, appear to be mixed in an 

 irregular way throughout the germ-mass. Gradually, however, certain of the 

 blastomeres take up definite positions, and, from what is known of their sub- 

 sequent history, can now be definitely stated to give rise to certain tissues. 

 The first of these rudiments to be recognised is the ectoderm ; this takes the 

 form of certain large blastomeres which migrate to the surface opposite the 

 placenta and there give rise to the ectoderm and its derivatives. Other large 

 blastomeres which remain nearer the placenta have been found to give rise to 

 the entoderm and possibly some mesoderin ; the latter layer, however, appears 

 to arise largely from tbe smaller blastomeres whicli, with the kalymmocytes, 

 make up the main mass of the germ. ED.] 



t [KouoTNEFF (No. XVIII.) expresses grave doubts as to the advisability of 

 this subdivision, and there can be no doubt from recent investigations that 

 the developmental differences between these two divisions have been greatly 

 exaggerated. ED.] 



