No. I.] CONTRIBUTION TO INSECT EMBRYOLOGY. 59 



it as exceedingly ancient and as well-developed before the 

 existing subdivisions of the Arthropoda were established. To 

 seek a homologue of the "dorsal organ" among existing 

 annelids may be regarded by some as a hopeless undertaking. 

 Still I would call attention to Apathy's observation ('88) on 

 Clepsine biocidata. The adult of this species has long been 

 known to possess a chitinous plate in the median line between 

 the head and the prasclitellum. Apathy finds that this plate is 

 the remnant of an embryonic sucking-disk, the glandular cells 

 of which secrete a bundle of byssus-like threads that harden 

 on contact with the water and serve to anchor the undeveloped 

 young to the ventral concavity of the mother-leech. A similar 

 organ is also found in the young of Clepsine heteroclita. It 

 is certainly no great step from this embryonic sucking-disk of 

 the Hirudinea to the Phyllopod "cervical gland" which is also 

 used as a sucker, and which Fritz Muller ('64) and Grobben 

 ('79) regard as homologous with the "dorsal organ" of the 

 Amphipoda. 



IV. The Envelopes and Revolution of the Insect Embryo. 



I. The Amnion ajid Serosa. 



The formation of two cellular envelopes, the amnion and se- 

 rosa, by a folding of the primitive extra-embryonal blastoderm, 

 is rightly considered one of the most characteristic features of 

 the Hexapod embryo. The envelopes are not, however, com- 

 mon to all insects. An amnion is completely lacking in the 

 Poduridae,^ and consequently the extra-embryonal blastoderm in 

 these forms is strictly comparable to the corresponding por- 

 tion of the blastoderm in Crustacea, Myriopoda, and Arachnida. 

 This is proved by the fact that it ultimately forms the definitive 

 dorsal body-wall. So far as our present knowledge extends, 

 the Apterygota may be regarded as Hexapoda Anamniota, and 



1 Lemoine ('87) describes a cellular " membrane amniotique " in Anurophoriis, 

 but he does not represent it in his figures and did not study it in section. I 

 therefore incline to doubt the correctness of his observation, especially as I can 

 find no traces of a cellular envelope in the Antirida egg, which on account of its 

 size is a far more favorable egg for study than that of Aiiurophorus. 



