THE FORMATION OF THE GERM-LAYERS. 



313 



bands unite first in the ventral middle line and only later in the 

 dorsal middle line. The food-yolk in this way comes to lie entirely 

 within the rudiment of the enteron (p. 336). 



In the Muscidae (and in a few other forms) the whole of the food-yolk is not 

 taken into the enteron, but a small amount remains in the body-cavity anteriorly 

 and posteriorly, and is there absorbed. 



Kowalevsky has already pointed out that it is only the median 

 portions of the lower layer which are separated as entodermal rudi- 

 ments at the anterior and posterior ends of the germ-hand by the 

 ingrowth of the stomodaeum and proctodaeum. The lateral parts 

 in these regions give rise to mesoderm. Kowalevsky has therefore 

 compared the formation of the germ-bands in the Insecta with their 

 formation in Sagitta. This view has received thorough support 

 from more recent researches made on Coleoptera (Heider, No. 38 ; 

 "Wheeler, Iso. 95). Even before the stomodaeal and proctodaeal 

 invaginations ap- 

 pear, the ento- 

 derm-rudiments 

 can here be seen 

 rising as a median 

 growth from the 

 base of the gas- 

 trula-furrow (Fig. 

 155), while the 

 lateral mesodermal 

 parts appear in the 

 form of lateral 

 sacs (Fig. 154 B 

 and D). The 

 separation of the 

 germ -layers in 

 the Insecta thus 

 resembles some- 

 what the condition 

 observed in Sa- 



■en ~ 



Fig. 154.— Diagrams illustrating the formation of the germ-layers 

 in Doryphora (after Wheeler). A, surface view. B, transverse 

 section through the anterior end of the germ-band at the 

 level of the lice aa. C, transverse section through the middle 

 of the germ-band corresponding to the line bb. D, transverse 

 section through the posterior end of the germ-band corre- 

 sponding to the line cc. bl, blastopore ; ec, ectoderm ; en', 

 anterior U-shaped entodenn-rudiment ; en", posterior U-shaped 

 entoderm-rudiment ; ms, mesoderm. 



gitta, where the 

 archenteron be- 

 comes divided by 

 the appearance of 



two folds into a median enteric rudiment and two lateral coelomic 

 sacs (Vol. i., p. 368). The chief peculiarity in the Insecta arises 



