CLEAVAGE AND THE FORMATION OF THE BLASTODERM. 



267 



recently been definitely maintained among others by Kowalevsky, 

 Blochmann, F. Schmidt, and Graber (No. 28) for Muscidae, by 

 \Vheeler for Doryphora, and by Heider for Hydrophilus. Patten, 

 on the contrary, has proved in connection with the egg of a Phry- 

 ganid (Neophylax), and Wheeler in connection with that of Blatta, 

 that in these forms all the "cleavage-cells" migrate to the surface 

 and take part in the formation of the blastoderm, so that there is 

 a stage at which the surface of the egg is covered by the blastoderm, 

 while the centre of the egg is devoid of nuclei. In these cases the 



Pig. 132. — Two diagrammatic sagittal sections through an insect-embryo to illustrate the 

 development of the embryonic envelopes. In A, the germ-band (/.-, k') is not completely 

 grown over by the amniotic fold. In B, the amniotic folds have united and completely cover 

 the germ-band, a, anterior, b, posterior pole of the egg ; v, ventral ; d, dorsal ; af, amniotic 

 fold; ah, amniotic cavity; am. amnion; do, food-yolk; ec, ectoderm; /;, cephalic end of 

 the germ-band ; V, posterior end of the germ-band ; s, part of the serosa derived from 

 the amniotic fold; .?', part of the serosa derived from the undifferentiated blastoderm; u, 

 lower layer. 



so-called yolk-cells only appear later, single blastoderm-cells wander- 

 in g again into the interior. As we shall see later, even in the 



DO ' 



forms first described, a secondary increase of yolk-cells takes place 

 by immigration from the blastoderm (or from the germ -band), 

 these forms, in which all the cleavage-nuclei reach the surface and 

 in which the immigration of the yolk-cells only takes place later, 



