TRICHOPTERA AND LEPIDOPTERA 351 



from the somatic mesoderm is composed of cells that are nearly as large 

 as the oenocytes but differ in being vacuolate. It is not until the 

 fourth day that fat and muscle cells are sharply differentiated, the 

 latter becoming striate a few hours before emergence of the larva. 



Toward the end of the third day the gonads are well advanced, the 

 genital ridges being formed laterally on the splanchnic mesoderm. In 

 the 84-hour-old embryo, they lie in the fifth abdominal segment (Fig. 

 312,fir). 



The neural furrow is well indicated at 45 hours. Three hours later 

 neuroblasts are distinctly differentiated from adjacent cells of ectoderm 

 in head and body segments. In its earlier stages the head exclusive of 

 the gnathal region is not segmented. The supraesophageal ganglion 

 formed by delamination from the ectoderm of the cephalic lobes at 

 65 hours shows by the presence of its neuropile a distinct tritocerebrum, 

 which is further identified by its commissure. The deutocerebrum is 

 obscured by the great development of the protocerebrum. Before the 

 end of the third day the divisions of the subesophageal ganglion, as well 

 as the ganglia of the thorax and abdomen, are well developed. As 

 development proceeds, the nerve cord separates from the adjoining 

 ectoderm. At 65 hours it is already quite free except along the median 

 furrow where ectoderm and cord merge. Six hours later the edges of the 

 ectoderm have fused along the middle line. The simple eyes, of which 

 five pairs develop, arise from clusters of moderately large ectodermal cells, 

 which appear early in the fifth day. 



Shortly before the mid-gut epithelium begins to develop, apodemes 

 appear in the head. The first, near the base of each mandible, is a deep 

 tubular invagination extending caudad, forming one of the anterior arms 

 of the tentorium. Two other shallower invaginations for the attachment 

 of the mandibular muscles appear in this head segment . A larger apodeme 

 on the side of the maxilla extending back into the labial head segment 

 forms one of the posterior arms of the tentorium. In the second maxillary 

 segment there are two pairs of invaginations : the inner pair becomes the 

 silk glands; the outer pair develops into the so-called " hypostigmatic 

 gland" of Toyama (1902). The invaginations for the silk glands (Fig. 

 307 B,silkg) are found just laterad of the neural groove in the labial 

 segment. By the sixty-fifth hour they extend back as simple, single- 

 layered tubes to opposite the second pair of thoracic legs ; by the eighty- 

 fourth hour they extend beyond the seventh abdominal segment. 



The " hypostigmatic gland" develops as an invagination near the 

 root of the second maxilla (labium). At 65 hours, the invaginated cells 

 are larger than those adjacent; the entire invagination is spherical in 

 form and gland-like in appearance (Fig. 307.6,^0, with the mouth of the 

 invagination directed cephalad. During the course of the next 10 hours 



