NO. 3 



INSECT HEAD SNODGRASS 



The mesoderm, and the associated mesenchyme, play an important 

 part in the organization of all the higher Metazoa, since they form the 

 internal organs that lie hetween the ectodermal covering of the body 

 and the endodermal epithelium of the alimentary canal. The meso- 

 derm is of particular importance in segmental animals because it is 

 in this layer that metamerism originates. Mesoblastic tissue is pro- 

 duced in a gastrulated embryo in two ways : First, in the form of 

 scattered cells proliferated from the inner surface of the invaginated 

 endoderm ; and second, in the form of cell layers. The scattered cells 



Pre 



B r/ Asd 

 Bp 



Ecd 



AMR 



Msd 



Ecd 



Fig. 5. — Gastrulation in insects. 



A, embryo of Lcptiuotarsa deciml'mcata with long gastrulation groove, or 

 blastopore {Bp), on ventral surface. (From Wheeler, 1889.) 



B, cross section through blastopore of embryo of Forficula, showing mesoderm 

 (Msd) formed by invagination of middle plate. (From Heymons, 1895.) 



C, anterior mesenteron, or endodermic, rudiment (AMR) formed at anterior 

 end of mesoderm (Msd) in embryo of honeybee. (From Nelson, 1915.) 



form a loosely coherent mesenchyme ; the cell layers constitute the 

 true mesoderm. The primitive mesoderm cells are given ofif from the 

 endoderm near where the latter joins the ectoderm, that is, just within 

 the lips of the blastopore (figs, i D,6, Msd). In the young annelid 

 larva, the mesoderm cells first form two lateral bands of tissue at the 

 posterior end of the body (fig. 7 D, Msd). Later, the extended meso- 

 derm tracts become excavated by a series of cavities, the coelomic 

 sacs, which mark the beginning of segmentation. In Pcripatiis (fig. 4), 

 likewise, two rows of coelomic sacs {Msd) are formed as paired 

 cavities in the mesoderm, which extends laterally between the ecto- 

 derm and the endoderm along the line of junction between these two 



