128 



MAINK ACRICL'LTURAI. KX I'KKI M liXT STATION. I9O3. 



increases in thickness al llic lime of the arrival of the nuclei. 

 This tliickenint;- seems to l)e (hie to tlie taking- u]) of the pro- 

 tophism wliich accompanies the nuclei in their mi,q;ration throug-h 

 the yolk. .\s the nuclei increase in nuniher they become 

 crowded and ])ress outward, forming- little protuberances on the 

 surface of the egg. Gradually, due to the increase of nuclei 

 within, the outside walls of the protuberances approach each 

 other and tinally fuse to form the cell walls of the blastoderm. 

 The inner wall of the cells appears later and seems to be formed 

 by the hardening of protoplasm at that point. 



The blastoderm extends rapidly over the surface of the yolk 

 from the point where the nuclei first came to the surface. 

 Before the blastoderm has come to cover the whole surface of 

 the yolk, it has commenced to thicken over a small area on the 

 ventral (concave) surface of the egg, a little in front of the pos- 

 terior end. This thickening increases in size and forms the 

 embryonic disc. In a short time a slight invagination appears 

 at the centre of the disc. The invagination deepens rapidly and 

 becomes directed toward the anterior end of the egg. The cells 

 of the anterior limb of the invagination increase in length and 

 thickness, while those of the posterior limb become very much 

 flattened. The invagination deepens by the addition of cells at 

 its inner end, and the space between the two limbs of the invagi- 

 nation, the amnionic cavity, becomes reduced to a narrow lumen 

 open to the surface at the point where invagination began. This 

 opening continues to grow smaller as the blastoderm folds 

 approach each other over the point of invagination. See Fig. 47. 



During the early part of the first day the folds of the blasto- 

 derm now marking the amnion and serosa come together over 

 the mouth of the invagination. The two folds fuse at the point 

 of contact, but the inner and outer layers soon separate from 

 each other. The amnion however continues to remain almost 

 in contact with the serosa so that there are only a few yolk gran- 

 ules between them at any time. The embryo, therefore, is of the 

 superficial type, such as had been described by Metschnikofif* for 

 Corixa. 



Sometimes before the closure of the invagination has taken 

 place there begins to be formed on the dorsal side ot the germ 



*Metsohnikoff E. Untersuchungen Uber die Embrj^ologie der Hemipteren. 

 Zeits. fur Wiss. Zool., Bd. XVl, 1886. 



