1870.] 13 [Packard. 



Embryology of Isotoma, a Genus of Poduridjl. By A. S. 

 Packard, Jr., M.D. 



■ The eggs were found laid singly or in masses on the damp under 

 surface of the bark of an apple tree. They are spherical, white, with 

 the chorion very transparent, and measure .0625 of an inch in diame- 

 ter. On the 25th of April many of the young had hatched out; they 

 continued to hatch until the 6th of May. 



In none of the eggs was I fortunate enough to observe the segment- 

 ation of the yolk, nor the formation of the blastoderm. Numerous 

 eggs, however, were observed, in which the blastoderm had not yet 

 been formed. In these, amid a mass of minute cells, floated four large 

 fat cells, measuring about one-fourth the diameter of the egg itself, 

 with numerous smaller cells measuring about one-fourth the diameter 

 of the largest cells just mentioned. The earliest stage observed was 

 that when the blastoderm has been resolved into the primitive band. 

 (Fig. 1.) At this time the primitive band becomes infolded, indi- 

 cating the cephalic lobes of the head. In a succeeding stage (Fig. 2) 

 the antennal, mandibulary and 1st maxillary segments, and the three 

 thoracic segments are indicated. The formation of the germ thus 

 far closely resembles that of the Phryganeidaa, as described and 

 figured by Zaddach. 



The next change is the closure of the body walls over the yolk, and 

 the appearance of the rudiments of the appendages, and the ceph- 

 alic lobes. At this time, also, the somewhat bilobate end of the 

 abdomen is formed, and also the rudiments of the future " spring," 

 consisting of a pair of tubercles, larger at this period than the rudi- 

 mentary antenna?. This fact is interesting, as I have observed in 

 other insects (Bombus, Vesjxz, Cicada, Aschna and Agriori) that these 

 were sternal outgrowths, and not articulated, and therefore, as I 

 supposed, not homologous with the legs and jointed appendages of 

 the head. This spring, therefore, partially represents the ovipositor 

 of the higher insects, the ovipositor originally consisting of three such 

 pairs of tubercles. At this period the "amnion" or " parietal em- 

 bryonal membrane," appears as a tough membrane surrounding the 

 embryo. 



In a succeeding stage the intestine is formed, and the rudiments 

 of the antennae and legs have greatly increased in size. Still 

 later the appendages begin to show traces of articulation, and the 



