536 KNOWER. [Vol. XVI. 



to grow into the elongated segmented embryo. From this time 

 a superficial germ-band would be constantly exposed to the 

 dangers mentioned. Hence the invagination, at this period, of 

 a part of the disc, resulting in the formation of the amnio- 

 serosal fold. 



The Termite and some of the Orthoptera (Stenobothrus, 

 Gryllus, etc.) have best retained this method of the formation 

 of the amnion. 



In other Orthoptera, the Libellulids, some of the Hemiptera, 

 and many other insects, the ancestral history is not so well 

 preserved. In these the amnion no longer closes over at the 

 earliest possible stage. Wheeler's figures of the germ-bands 

 of Blatta and Doryphora (25), Graber's of Lina (9), Heider's of 

 Hydrophilus (13), and Weismann's of Chironimus (24) illustrate 

 its usual late closure. 



The Libellulids and some of the Hemiptera retain to a 

 decided degree ancestral characters, but the much-retarded 

 closure of the amniotic cavity, and the presence of the so-called 

 secondary "head-fold," together with the marked secondary 

 " immersed " position of the germ-band, render these forms 

 less typical examples of the probable primitive method of 

 development. {Refer to the disaission of the ongin of the 

 amnion in insects, in the last divisiojt of this paper, for ftirther 

 consideration of these questions) 



The Origin of the Mesoderm in Insects. 



Recently the origin of the under-layer in what are regarded 

 as the most primitive insects, the Orthoptera, has been care- 

 fully studied by two well-known investigators who have reached 

 quite contradictory results. 



Wheeler (26), in his " Contribution to Insect Embryology," 

 has devoted considerable space to a review of the question. His 

 conclusion is expressed in these words: "It follows from the 

 observations here recorded, fragmentary as they are in many 

 respects, together with Graber's observations on Stenobothrus, 

 that the Orthoptera can no longer be regarded as hors de ligne, 

 so far as the formation of their germ-layers is concerned. In 



