354 



DEVELOPMENT OF THE MESOBLAST. 



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. 



Gephyrea. 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 meso- 

 blast might be described as originating at the lips of the blastopore (primi- 

 tive streak). 



6. The mesoblast is derived from both germinal layers. 



Tracheata. Araneina (fig. 214). 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 accurate ; and that the derivation of part of the mesoblast 

 from the yolk-cells is not to be interpreted as a derivation from the 

 hypoblast. 



Amniota. The derivation of the mesoblast of the Amniota from both 

 the primary germinal layers is without 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 Ccelenterata would lead us to expect that the mesoblast 



FIG. 211. 



A. 

 B. 



SECTIONS OF AN AMPHIOXUS EMBRYO AT THREE STAGES. 



(After Kowalevsky.) 

 Section at gastrula stage. 

 Section of a somewhat older embryo. 

 C. Section through the anterior part of still older embryo. 

 np. neural plate; nc. neural canal; mes. archenteron in A, and mesenteron in B 

 and C ; ch. notochord ; so. mesohlastic somite. 



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 shew that there are a 

 greater number of instances in which the mesoblast has a mixed 

 origin than might be supposed from the above summary. 



