292 



DEVELOPMENT OF THE MESO BLAST. 



It is derived from the lateral and ventral 



Enteropneusta (Balanoglossus) — The body cavity is derived from 

 two pairs of alimentary diverticula, the walls of which give rise to the 

 greater part of the mesoblast. 



Chordata — Paired archenteric outgrowths give rise to the whole me- 

 soblast in Amphioxus (fig. 211), and the mode of formation of the meso- 

 blast in other Chordata is probably secondarily derived from this, 



3. The cells which will form the mesoblast become marked out 

 very early, and cannot be regarded as definitely springing from 

 either of the primary layers. 



Turbellaria — Leptoplana (fig. 212), Planaria polychroa (?). 

 Chsetopoda. — Lumbricus, &c. 

 Discophora. 



It is very possible that the cases quoted under this head ought more 

 jtroperly to belong to group 1. 



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 ty])es without a metamorphosis. 

 Mollusca — Scaphopoda. 

 cells of the hypoblast. 



Qephyrea — Phascolosoma. 



Vertebrata In most of the Ichthyopsida the mesoblast is derived 



from the hypoblast (fig. 213). In some types {i.e. most of the Amniota) 

 the mesoblast might be described as originating at the lips of the blasto- 

 pore (primitive streak). 



6. The mesoblast is derived 

 from both germinal layers. 



Tracheata — Araneina (fig. 214). 

 It is derived partly from cells split ofl[' 

 from the epiblast and partly from the 

 yolk-cells; but it is probable that the 

 statement that the mesoblast is de- 

 rived from both the germinal layers 

 is only formally accurate ; and that 

 the derivation of part of the mesoblast 

 from the yolk-cells is not to be in- 

 terpreted as a derivation from the 

 hypoblast. 



Amniota — The derivation of the 

 mesoblast of the Amniota from both 

 the primary germinal layers is with- 

 out doubt a secondary process. 



The conclusions to be drawn 

 from the above summary are by no 

 means such as might have been 

 anticipated. The analogy of the 



ME 



Bl]l. Ptd. 



Fig. 210. Longitudinal section 

 through an embbyo of cucumabia do- 

 liolum at the end of the foubth dat. 



Vpv. vaso-peritoneal vesicle ; ME. 

 mesenteron; Bijp., Ptd, blastopore, proc- 

 todeeum. 



