PHYLLOPODA. 125 



It would be interesting to learn something of the formation of the mid-gut, 

 and of the part here played by the food-yolk in the eggs of those Cladocera or 

 Branchiopoda in which it abounds. Moina is a striking exception to the rule 

 in being poor in yolk. In young BrancMpus larvae all the tissues, even the 

 ectoderm, appear to be permeated with granules of food -yolk (cf. Glaus, 

 No. 9). 



In Daphnia similis, according to the recent researches of Lebedinsky 

 (No. 11a), a blastoderm, of equal thickness throughout and completely covering 

 the egg, is first formed by superficial cleavage. This only thickens later at 

 points where the cells become elongated in the region of the cephalic lobe and 

 on the ventral side of the egg. The formation of the germinal layers is com- 

 menced by the appearance of a very shallow depression (blastopore), from which 

 point immigration of amoeboid cells into the yolk takes place. The latter 

 represent the meso-entoderm. While the mesoderm-cells become arranged into 

 two symmetrical bands running forwards from the blastopore (mesoderm-bands), 

 the entoderm forms a solid strand, in which a cavity develops at a later stage. 

 Not all the entoderm-cells, however, take part in the formation of this mid-gut 

 strand. ' ' A few of them form a covering to the food-yolk, and give rise to 

 two large symmetrically-placed provisional hepatic vesicles." (?) 



In Moina, the breaking-up of the mesoderm into somites and 

 the development of a true coelom has not been observed. In the 

 Branchiopoda, where the formation of the germ-layers is not yet 

 known, we must fall back on observations recorded in connection 

 with the larval stages of Artemia and Branchipus. In the earliest 

 Nauplii of Artemia, there is, according to the figures of Nassonow 

 (No. 13), a temporary development of paired coelomic vesicles. In 

 Branchipus, on the contrary, whose youngest or Metanauplius stage 

 has been carefully investigated by Claus (No. 9), the process is 

 different. Here the mesoderm in the region of the actual Nauplius 

 segments and of the terminal segment has already become modified 

 for the formation of organs, and has attained definite histological 

 differentiation. The same is the case with the splanchnic layer 

 along the whole of the alimentary canal (Fig. 88 A, sp, p. 179). 

 In those segments which are interposed between the mandibular 

 and the terminal segment, and are found in the act of appearing, 

 the somatic layer bears a more embryonic stamp. It is here 

 arranged in paired mesoderm-bands, whose cells appear to be seg- 

 mentally arranged in a definite manner. This arrangement is due 

 merely to a regular grouping of the mesoderm-cells, which to some 

 extent recalls the arrangement described below (p. 137) in connection 

 with the Isopoda. In the most posterior regions of the body, the 

 niesoderm-bands are united to form a plate lying below the intestine, 

 and here is found the budding zone, from which proceeds the 

 formation of new segments. Grobben thought himself justified in 



