THE DEVELOPMENT OF EGGS OF THE SCREW-WORM FLY COCHLIOMYIA HOMINIVORAX (COQUEREL) (DIPTERA: CALLIPHORIDAE) TO THE BLASTODERM STAGE AS SEEN IN WHOLE-MOUNT PREPARATIONS

¹ Entomology Research Division, Agric. Res. Serv., USDA, Mission, Texas

1. Development of screw-worm eggs from the first meiotic division to blastoderm formation was studied from whole-mount preparations. Both meiotic divisions were completed by 7-8 minutes. Syngamy at 14-15 minutes was quickly followed by the first cleavage division. The first 8 cleavages took place within the interior of the eggs, and the last of these occurred approximately one hour after egg deposition. After the 8th division, most cleavage nuclei moved near the egg surface to form the blastema by the end of about 1 hour and 10 minutes. This movement was followed by four more divisions of the blastema nuclei. The last of these divisions was underway, or had been completed, by the end of the second hour. There was then a prolonged interphase period, during which cell membranes formed around the blastema nuclei to become the definitive blastoderm.

2. The first 8 cleavages were synchronous, but for the last four divisions an anterior-posterior gradient was evident. During the 11th division, and especially during the 12th division, there was also an accompanying mitotic gradient proceeding from the posterior end of the egg.

3. After the 8th cleavage some nuclei remained behind to form the yolk nuclei or vitellophags. Also, the pole cells were set aside after the 9th division but prior to the 10th.

4. A low order of polyspermy was found in most eggs, but many of them received only a single sperm. Most unfertilized eggs completed at least the first meiotic division, but very few of them achieved the first cleavage division and none developed further than this stage.