74 CLAVPOLE. [Vol. II. 



together with a pair on the intercalary segment which takes 

 part in the formation of the adult mouth. These are homolo- 

 gized with the second pair of Crustacean antennae. Yolk is 

 found enclosed with reproductive cells, causing their very- 

 rapid development. Anurida agrees with the rest of the 

 Collembola in showing characters allying it strongly with the 

 lower arthropods. 



Summing up the present state of knowledge regarding 

 Apterygote embryology, it is found that at least fourteen species 

 of Collembola and three of Thysanura have been studied with 

 more or less care. This work confirms by its results the opin- 

 ion that the Apterygota possess truly primitive characters and 

 also show transitions to the higher and lower Arthropoda. 

 Cleavage among the Collembola, as far as determined, shows 

 many types : equal holoblastic, unequal holoblastic, holoblastic 

 becoming superficial, and truly superficial ; while on the other 

 hand the Thysanura show only the superficial type, whether the 

 eggs are spherical or oval. It is unfortunate that in most 

 cases the size of the eggs is not given, and in many instances 

 the method of cleavage is unknown. Still a comparison of the 

 three available forms is instructive. 



Anurida. Spherical, .27 mm. in diameter. Cleavage holoblastic 

 becoming superficial. 



Campodea. Spherical, .4 mm. Superficial cleavage. 

 Lepisma. Oval, I. mm. Superficial cleavage. 



The apparent discrepancy between the two sizes given for 

 the eggs of Anurida by Ryder and myself is readily explained, 

 by the fact that the measurements were taken at different 

 stages. There is a marked increase in size during develop- 

 ment. No early stages are described for Isotoma, whose size 

 is the smallest yet recorded (.15 mm.), but some of Packard's 

 so-called gerni-band figures suggest strongly that they are pos- 

 sibly stages showing the first cleavage plane appearing. Enough 

 is given in this short series to indicate a regular increase in the 

 size of the Qgg and a pari passu loss of holoblastic cleavage in 

 passing up the scale of apterygote insects. The only trace of 

 the total cleavage remaining in Tepisma is shown in the yolk 



