NO. 3 EMBRYOLOGY OF FLEAS KESSEL 3/ 



all insects. An examination of the literature reveals descriptions of 

 gastrulation in insects by the four following methods : 



i. The lower layer is formed by an emigration or proliferation of 

 cells from the blastoderm along the median line of the ventral plate. 

 No obvious median groove is produced to assist in this process ; in 

 some instances all indications of such a groove are lacking. Korschelt 

 and Heider (1899) discount the reports of the existence of this type, 

 apparently because of their conviction that a tube, or groove at least, 

 must be present as the remnant homolog of the gastrula-tube of other 

 animals. There is no doubt, however, that such a method of lower 

 layer formation actually exists in some insects. It has been observed 

 by Heymons (1897) in Lepisma and by Uzel (1898) in Campodea. 

 It is also described for the Collembola by Claypole (1898), Phil- 

 liptschenko (1912), and Weber (1933). In addition, it has been 

 reported in several of the Pterygota by a number of workers including 

 Korotneff (1885) for Gryllotalpa, Wheeler (1889) for Blatta, and 

 Hagan (1931) for Hesperoctenes. 



2. The lower layer arises by an emigration of cells from the blasto- 

 derm along the median line of the ventral plate. In this case a distinct 

 groove is formed, but its lips are not approximated to form a tube. 

 This method has been described by Patten (1884) for Trichoptera, 

 by Will (1888) for Aphidae, and more recently by Gambrell (1933) 

 f or Simulium and by Auten (1934) for Phonnia. 



3. There occurs an actual invagination of the ventral midline blasto- 

 derm to form a groove which changes into a closed tube by the 

 approximation and fusion of its lips. This type of lower layer separa- 

 tion has been reported in several of the higher insects such as Hydro- 

 pJulus (Kowalevsky, 1871, Heider, 1885 and 1889), Leptinotarsa 

 (Wheeler, 1889), Chalicodoma (Carriere, 1890), CallipJwra (Kowa- 

 levsky, 1886), and Calendra (Wray, 1937). 



4. The middle portion of the ventral plate becomes separated from 

 the lateral blastoderm. This middle plate then sinks inward while the 

 lateral plates grow together and fuse over its outer surface. Like the 

 third type described, this method has been reported only for certain 

 higher insects. Among them are Apis (Nelson, 1915), Spliinx (Kowa- 

 levsky, 1871), and Pieris (Bobretzky, 1878). 



In fleas the lower layer is produced by a combination of the first 

 three methods listed above. In the anterior third of the ventral plate, 

 including the brief section between the anterior mesenteron rudiment 

 and the anterior amnio-serosal fold, there is a simple emigration of 

 cells from the blastoderm, without the formation of a perceptible 



