GERMINAL LAYERS OF THE EMBRYO. 
267 
4. The mesoblast cells are split off from the epiblast. 
Nemertea . — Larva of Desor. The mesoblast is stated to 
be split off from the four invaginated discs. 
5. The mesoblast is split off from the hypoblast. 
Nemertea . — Some of the types without a metamorphosis. 
Mollusca. — Scaphopoda. It is derived from the lateral and 
ventral cells of the hypoblast. 
Gephjrea. — Phascolosoma. 
Vertebrata . — In most of the higher Vertebrata the meso- 
blast is derived from the hypoblast (fig. 16). In some types 
(f. e, most of the Icthyopsida) the mesoblast might be 
described as originating at the lips of the blastopore. 
Fig. 13 . — Longitudinal Section through an Embryo ofCucumaria doHolum at 
the end of the fourth day. Vpv. vaso-peritoneal vesicle : ME. mesenteron ; 
Blp.y Ptd. blastopore, proctodaeum. 
6. The mesoblast is derived from both germinal layers. 
Tracheata . — Araneina (fig. 17). It is derived partly from 
cells split off from the epiblast and partly from the yolk-cells ; 
but it is probable that the statement that the mesoblast is 
derived from both the germinal layers is only formally accu- 
rate ; and that the derivation of part of the mesoblast from 
the yolk-cells is not to be interpreted as a derivation from 
the hypoblast. 
The conclusions to be drawn from the above summary are 
by no means such as might have been anticipated. The 
analogy of the Coelenterata would lead us to expect that the 
mesoblast would be derived partly from the epiblast and 
partly from the hypoblast. Such, however, is not for the 
most part the case, though more complete investigations may 
