6 J. T. PATTERSON 



2. STRUCTURE, MATURATION, AND FERTILIZATION OF THE EGG 

 a. The freshly laid egg 



The freshly laid egg is a pear-shaped cell, surrounded by a 

 very thin but tough membrane, which is probably a true cho- 

 rion (fig. 1). The broad or posterior end of the egg corresponds 

 to the vegetative pole of other eggs. It is from this region 

 that the true embryonic cells are formed by the process of cleav- 

 age. The anterior end of the egg gradually narrows down 

 and finally terminates in a finger-like process, which is char- 

 acteristic of the eggs of many parasitic hymenoptera. As 

 development progresses the content of this process is gradually 

 taken into the egg proper, so that in later stages it is no longer 

 seen. 



As a matter of fact, the different eggs vary greatly both in 

 shape and in size (figs. 1 to 13). The average unfixed egg meas- 

 ures about 155 /i by 60 ju in its major axes. Its content consists 

 of a very finely granular protoplasm, in which are found a few 

 yolk or oil spherules (fig. 34). In the fertilized egg there are 

 found three distinct bodies, the oocytic nucleus, the sperm, and 

 the so-called nucleolus. 



In the freshly deposited egg the nucleus is an oval-shaped body 

 in which the chromatin appears as elongated threads running 

 more or less parallel with the long axis of the nucleus (figs. 1, 2). 

 It is always situated well toward the anterior end of the egg, 

 and usually near the base of the finger-like process. Martin 

 ('14) and Hegner ('14, '15) have traced out the origin of the 

 nucleus in two species of polyembryonic hymenoptera (Ageni- 

 aspis fuscicollis, and Copidosoma gelechiae). According to the 

 accounts of these writers, it has a rather remarkable history. 

 In Copidosoma the young oocyte in its nurse chamber has a 

 very large nucleus, in which is found an irregular, deeply stain- 

 ing mass of chromatin. The cytoplasm forms a very thin layer 

 about the nucleus. During the growth period the cytoplasm 

 increases rapidly, while the nucleus enlarges but very little. 

 Later the chromatin loosens up and forms a spireme, which 

 finally breaks up to form thin, irregular-shaped chromosomes. 



