39i 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1925 



THE GERM LAYERS 



Soon after the werm band is formed, its middle part begins to 

 sink inward, and may become separated from the part to either 

 side by a slight ridge. The germ band thus becomes divided into 

 two lateral plates (fig. 13 A, LP, LP) and a middle plate {MP). 

 The middle plate is destined to form the strictly internal organs 

 of the future insect, and it continues to sink inward, in some cases 

 forming a deep median groove (B), which is then converted into 



Meso E'ct 



c 



Ect Meso 



F 



-Ect 



Fig. 13. — Showing diagrnniuiatically tlirof methods of formation of mesoderm 



l;..vcr ill iiiwct.-; 



A, cross-section of egg with geim hand differentiated into lateral plates (LP, 

 LP) and middle plate (MP). H, later stage of same with middle plate rurved 

 in to form a tuliular groove, edges of lateral plates coming together helow 

 it. C, still later stage, with edges of lateral plates united, forming the 

 ectoderm (Ect), and middle plate spread out above the latter as internal 

 layer oT cells, the mesoderm (Meso). D, K, second method of mesoderm 

 formation : middle plate, separated from edges of lateral plates, becomes meso- 

 dernn (Meso) wlioi) lateral plates unite beneath it. F, third method, in 

 which mesoderm (Me.so) is formed of cells given off from inner ends of 

 middle plate cells. 



a sunken tube by the approach and union of (he edges of the 

 lateral plates beneath it. Finally, this middle plate foruis a solid 

 ma.ss of cells which flattens out into a ventral layer (C, Meso) 

 between the yolk and (he outer layer {Kct) which is now con- 

 tinuous beneath it. The process of the formation of (he inner germ 

 layer differs somewhat from the above type in certain other insects. 

 In some, for exauiple, (he edges of the lateral plalcs .separa(e from 

 the middle plate (fig. 13 D) and then giow together beneath it 

 (E), (he middle pla(e being (lius cut off from the lateral ])latcs 

 and, b}' a more direct course, converted into the inner germ layer. 



