No. 2.] THE EMBRYOLOGY OF THE APTERYGOTA. 71 



Grassi^ gives the first mention of thysanuran development. 

 He describes three features in the developmental processes 

 of Japyx. The cleavage is distinctly superficial ; an amnion 

 exists, and also a "dorsal organ." 



Ryder ^ published studies on Anurida maritima Guen., giving 

 the following results. After the formation of the germ band 

 two membranes invest the embryo, the inner one being cre- 

 nated. The embryonic area forms a nearly complete belt sur- 

 rounding the Q.gg, and seven pairs of appendages can be made 

 out: antennae i, mandibles i, maxillae i, thoracic legs 3, and 

 the collophore on the first abdominal segment. A rudimentary 

 spring is reported as still visible on the fourth abdominal seg- 

 ment of the hatched larva, but no trace remains in the adult. 



Wheeler^ gives the first account of section views of an 

 apterygote insect. He shows the existence of an intercalary 

 segment with appendages in the head of Anurida maritima, 

 placed between the mandibular and antennal segments. The 

 "dorsal organ" is shown in section and is definitely homolo- 

 gized with the "indusium " of Xiphidium. 



Later studies all include internal structure as seen in sections. 

 In 1896 Heymons'^ published a short account of his work on 

 Lepisma saccharina, the highest of the Thysanura. The eggs 

 of this species are oval and about i mm. in their longest diame- 

 ter ; cleavage is distinctly superficial, and an extremely small 

 germ band early appears. This is found to sink immediately 

 into the yolk, still, however, retaining its connection with the 

 serosa, the extraembryonic part of the blastoderm, by a thin 

 membrane, the amnion. As the embryo sinks in, the amniotic 

 cavity becomes large and distinct, but always retains connection 

 with the outside by the open amnion pore. The amnion is 

 hence never constricted from the serosa. By later growth of 

 the germ band the pore is opened and the amnion is retracted 



* Grassi, B., " I Progenitori degli Insetti e dei Miriapodi I'Japyx e la Campo- 

 dea," Atti accad. Givenia Sci. Nat. in Catania. (3), vol. xix. 1885. 



^ Ryder, J. A., " The Embryology of Anurida Maritima Guen.," Anier. Nat. 

 Vol. XX. 18S8. 



6 Wheeler, W. M., " A Contribution to Insect Embryology," /M<r«. of Morph. 

 Vol. viii. No. I. 1893. 



■^ Heymons, R., " Ein Beitrag zur Entwicklungsgeschichte der Insekten Apter- 

 ygota," Sitz. Berickte Acad. Wiss. Berlin. 1896. 



