No. I.] CONTRIBUTION TO INSECT EMBRYOLOGY. 63 



Conditions similar to those to which the insect germ-band is 

 subjected during its younger stages are often present in the 

 ova and young of other animals, and would be expected to lead 

 to the formation of structures similar to the insect amnion. 

 And this is found to be the case. A hasty glance through the 

 animal kingdom at once suggests a number of parallel instances : 

 the invagination from which the Cestode head develops in the 

 Cysticercus ; the similar invagination in the larval Gordiid ; the 

 origin of the Nemertine in the Pilidium; the formation of the 

 definitive trunk in Aulastoma, according to Bergh ('85); the 

 development of the trunk and Scheitelplatte in Sipimciihis , 

 according to Hatschek ('84); the formation of the young 

 Spatangid in the Pluteus, according to Metschnikoff, and the 

 somewhat similar conditions in the development of the An- 

 iedon, according to Barrois ('88); the formation of the trunk 

 in the Actinotrocha of Phoronis (E. B. Wilson '8I); the de- 

 velopment of the Polyzoan within the statoblast (Oka '91 ; 

 Davenport '91). I need hardly say that the development of 

 the amnion and serosa in vertebrates is a strictly analogous 

 case. A case still more to the point, because occurring in 

 the Insecta, is the formation of the imaginal disks. In this 

 process we have all gradations till we reach the extreme in 

 Miisca, where the hollow disks whose inner walls bud forth 

 the imaginal appendages are almost completely abstricted from 

 the original hypodermis. The resistance of the chitinous cuti- 

 cle of the larva in causing the invagination of the disks admits 

 of easy observation. It certainly cannot be claimed that in all 

 the different forms here enumerated genetic relationship lies at 

 the bottom of the mutual agreement in the methods of form- 

 ing the trunk or certain organs. On the contrary, everything 

 o-oes to show that these similar methods in widely separated 

 groups have been independently acquired under the stress of 

 similar developmental conditions. 



Perhaps the most difficult point to explain in the view here 

 advanced, is the complete abstriction of the amnion from the 

 serosa in nearly all insects. It is more natural to suppose 

 that the inner envelope would remain continuous with the 

 outer, so that the embryo could the more readily be everted 



