OOGENESIS IN PHILOSAMIA 25 
as before. At the end of the fourth differential mitosis, there 
are 15 nurse cells and one egg cell, which enters the resting 
stage. The spindle remains of the nurse cells join that of the 
egg, forming a protoplasmic bridge between them. Giinthert 
believes that differential divisions occur in many animals; 
and that the ‘accessory body’ described by Buchner (’09) in 
Gryllus is merely a ‘chromatic mass’ indicating a differential 
mitosis. The origin of nurse cells from the egg by a process of 
budding, as described by Will, is probably to be interpreted in 
the same way. In the later nurse cells of Dytiscus Giinthert 
finds tetrads which subdivide regularly several times, freeing 
thousands of granules in the nucleus. When the nuclear wall 
breaks down, they migrate into the cytoplasm, where they increase 
by division, eventually entering the egg. 
Orthoptera. Buchner (’09) figures a leptotene stage in Gryllus 
after the oogonial divisions, followed by a diplotene stage dur- 
ing which the ‘accessory’ is much vacuolated. The probable 
significance of this body has been referred to above. After the 
diplotene stage the threads shorten into rods and tetrads. No 
further development of the egg is given. 
SUMMARY 
1. In Philosamia cynthia the 13 bivalent chromosomes of 
the late prophase all divide in both maturation divisions. 
2. The male and female pronuclei at the time of their union 
each contain 13 chromosomes, making the somatic number 26, 
which is found in the nuclei of the blastoderm. 
3. On account of the great variations in the size of the 
chromosomes in the metaphase and anaphase plates, there is 
no conclusive evidence for either the presence or the absence of 
an X Y-pair of chromosomes. 
4. In the oogonia no differential divisions occur. The germ 
cells all appear similar through the presynaptic and synizesis 
stages. 
