344 J- T. PATTERSON. 



toplasm surrounding a group of nuclei soon rounds off and the 

 primordial embryo with its surrounding layer lies free within the 

 more fluid contents of the central region of the egg (Fig. 19 A}. 

 The more usual condition is for the spherical mass to remain 

 attached at one side to the peripheral layer of the granular pro- 

 toplasm (Fig. 19 B, P.E.). Eventually all of the embryonic 

 nuclei thus become included in these spherical masses of pro- 

 toplasm, and thus become isolated as primordia of the embryos. 



The condition at the close of the formation of the primordia 

 is shown in Fig. 21. This specimen was found in a series of 

 sections of a 3 mm. caterpillar, taken June 15, 1914. In the 

 median section it measures 113 M by 203 /i, and runs through 40 

 sections (280^1). It lies in the middle portion of the body 

 cavity, to one side of the intestine, which on account of the size 

 of the polygerm is pushed out of place. As compared with the 

 preceding polygerms this one is very much larger and shows a 

 number of important changes. The nucleated membrane has 

 become much thinner and its nuclei are arranged more or less 

 into a single layer. The adipose tissue is being laid down on the 

 outer surface of the membrane. The most important change, 

 however, has occurred in the embryonic masses themselves. 

 The protoplasm which surrounds a group of nuclei is differenti- 

 ated into two distinct regions. The central part, crowded with 

 nuclei, stains somewhat lighter than the peripheral zone, which 

 forms a relatively dense layer about the central core (Fig. 21, 

 P.E.). There are still a few nuclei which have not as yet been 

 surrounded by the dense layer, but this stage marks approxi- 

 mately the end of the division of the germ into separate embryos. 



(e) Growth of the Polygerm and the Formation of the Primary 

 Divisions or Masses. Upon the completion of the primitive 

 embryos, the polygerm grows very rapidly. It first extends in 

 the direction of its long axis, soon transforming into an elongated 

 cylindrical structure. One specimen showing this condition 

 measures in section 148 /j. by 430 yu. It never becomes an elon- 

 gated tube as does the polygerm of Ageniaspis. During this 

 growth the adipose tissue is laid down in the form of a thick 

 layer about the polygerm. One of the easiest ways in which 

 to find a polygerm of this and later stages is to examine the large 



