216 THE entomologist's record. 



changes its position. Very little further change in position takes place, 

 the embryo by this time occuj^j'ing all the available space in the egg. 



12. — Keversal of position of embryo in EGfis. — In dealing with 

 the embryonic development of Toririx ferrugana, I have shown that, 

 during the first stages of development, the ventral side of the embryo 

 is external, or lies along the convex side of the egg, development 

 commencing (as is usual in the Ai'ticulata and Vertebrata) on the 

 ventral side of the insect, and that, as development and the growth of 

 the segments proceed, the embryo, on account of the turning of the 

 anal segment and its gradual upward movement, and that of the grow- 

 ing segments behind it along the venter, changes its position, the ventral 

 part of the embryo gets turned towards the centre of the egg, whilst 

 the dorsal part is turned towards the outside. 



Dr. Osborne (E.M.M., vol. xix., pp. 99-100), writing upon the way 

 in which this reversal of position is brought about, says : — " How it 

 gets into this position, if it develops in the usual Arthropod way, is a 

 point which I have only seen adverted to by Kowalevski. Speaking 

 of the development of Sphiiix popnli and Gasiropaclia pini, he says 

 (" Embryolog. Studien an Wiirmern und Arthropoden." Memoires dc 

 V Acad. Imp. des Sciences de St. Petersburg, Series vii., Tom. xvi.. No. 12, 

 p. 56) : — ' Wenn der Eiicken schon gebildet ist, biegt sich das Schwan- 

 zende des Embryo auf die Bauchseite und zwar so, wie wir schon beini 

 Hydrophihts gesehen haben. Dem Hinterende folgend, dreht sich der 

 ganze Embryo so, dass er jetzt der ihn noch bedeckenden serijsen Hiille den 

 Kiicken zuwendet, und die Extremitiiten erscheinen nach innen gerichtet. 

 In diesem Zustande, mit fast vollstiindig ausgebildeten Organen, 

 bleibt der Embryo vollstiindig in dcm ihn umgebenden Dotter, den er nun 

 vermittelst der unterdessen voUstandig ausgebildeten Mundorgane zu 

 verschlucken beginnt * * * die * Larve liegt [jetzt] schraubenfurmig 

 auf der Bauchseite Zusammengerollt bis sie das Chorion zerreisst und 

 ins Freie gelangt.' The embryo of the sawfly, Zaraea fasciata, does not, 

 at any rate, get into the loop position by any molar movement of this sort. 

 When the })Osterior end of the growing embryo has reached the remote 

 end of the egg, it is bent ventrally on itself, and so grows forAvards till 

 the tail comes in contact with the head. As the length of the embryo 

 still continues to increase, the head is withdrawn to about the middle 

 of the straight or upjDcr side of the egg, and the larva about to hatch 

 lies in a spiral, Avith the tail opposite the head on the other side of the 

 body. It turns its sharp mandibles towards the shell, bites at it and 

 draAvs it in till it is pierced and, by means of a foot thrust through the 

 ojiening, draAvs the flexible chorion still more Avithin the power of the 

 mandibles, Avhich soon eifect an opening large enough for its escape. 

 This ingroAvth A^entrally of the caudal end of the embryo appears to be 

 not uncommon in the Arthropoda, Avhere the length of the embryo 

 exceeds that of the shell, and occurs even in the case of the globular 

 egg of Astncus, as described by Huxley {The Cray-fish, p. 2U3). In 

 the case of an embryo making such a rcA'olution in the egg as that 

 described by KoAvalevski, the head Avould occuijy two different positions 

 in the same end of the egg, relativel}^ to two 02:)iD0site sides before and 

 after the revolution. The egg of Rumia crataegata would be specially 

 favourable for making this obserA'ation ; the shell at the cej^halic end 

 being distinguished by an ellii)Soidal ridge : the pointed end of the 

 ellipsoid corresponds Avith the position of the head of the larva just 



