EARLY CHANGES IN THE OVUM. 231 



In the Diptera, as in the Echinodermata, there is a blastula 

 stage. The blastoderm at this period is a closed vesicle con- 

 sisting of a single layer of cells, which only differs from the 

 echinoderm blastula in the abundant food-yelk which it con- 

 tains. 



If the views which I have adopted are correct, in both cases 

 there is a total segmentation of the germ-yelk, and the archen- 

 teron is developed by an invagination of the blastoderm — 

 gastrulation — whilst the mesoderm in both is developed by a 

 pair of diverticula, originating from the hypoblast, in close 

 proximity to the original blastopore, which eventually surround 

 the archenteron. In the Muscidse, and in some Echinoderms, 

 Echinus, the definitive anus is developed as a distinct involu- 

 tion of the epiblast, and the blastopore is closed before the 

 proctodeum comes into relation with the archenteron ; more- 

 over, in both the greater part of the wall of the original 



108. Will, L., 'EntwickelungsgeschichtederViviparen Aphiden.' Spengel's 

 Zool. Jahrbuch. Abth. f. Anat. u. Ontog., Bd. iii., 1888. 



109. BiJTscHLi, O., 'Bemerkungen iiber die Entvvickelungsgeschichte von 

 Musca.' Morph. Jahrbuch, Bd. xiv., 1888. 



110. VOELTZKOW, P. A., ' V^orliiufige Mittheilung iiber die Entwickelung 

 im Ei von Musca Vomitoria.' Zool. Anzeiger, n. 278, 1888. 



111. VOELTZKOW, p. A., ' Entwickelung im Ei von Musca Vomitoria.' 

 Arbeiten aus dem Zool. Zoot. Inst, zu Wiirzburg, 1889. 



112. VOELTZKOW, p. A., ' Melolontha Vulgaris.' Ein Beitrag zur Ent- 



vvickelungsgeschichte im Ei bei Insecten. Arbeiten aus dem Zool. 

 Zoot. Inst, zu Wiirzburg, 1889. 



113. Choloukvoskv, N., 'Studien zur Entwickelungsgeschichte der In- 

 secten ' (Blatta Germanica). Zeitsch. f. w. Zool., Bd. xlviii., 1889. 



114. Gkaisek, v., ' Vergleichende Studien iiber die Embryologie der 



Insecten und insbesondere der Musciden.' Denkschrift, K. Akad. 

 Math. Naturwissen CI. Wien., Bd. Ivi., 1889. 



This work consists of 58 pages of letterpress, illustrated by 10 

 plates, and is the most complete work on the subject extant. It con- 

 tains an extensive bibliography. 



115. Graber, v., 'Vergleichende Studien am Keimstreif der Insecten.' 

 Denkschrift K. Akad. Math. Naturwissen CI. Wien. Bd. Ivii., 

 1890. 



This work consists of 113 pages of letterpress, illustrated by 12 

 plates. It is an epoch-making work on the embryology of insects. 

 The bibliography appended gives references to 82 memoirs on the 

 subject. 



16 



