42 ATLAS OF BIOLOGY. 



FIG. VIII. A small portion of a thin transverse section across the ischiopodite of the 



third maxillipede. 



The pore-canals are drawn in the upper half of the figure alone ; they are closer together 

 than here represented. D. 3. 



FIG. IX. Portion of a tangential section of the same. 

 These sections were ground down on a rough surface. See Appendix H. 

 The appendage chosen for the above is an instructive one, as all gradations of the 

 exoskeleton, from the seta to the cutting tooth, can readily be obtained in the same slice. 



FIG. X. The excretory organ of the left side, dissected from without, a portion of its 

 duct of communication with the exterior having been opened up. x 2. 



Wassiliew (36) states that the green gland consists of an unbroken tube of three 

 segments. 



FIG. XI. The male reproductive organs, drawn from above. 



The right limb was straightened out to show the whole course of the vas deferens, a 

 portion of which was opened up. x 3. 



FIG. XII. A ripe spermatozoau. D. 3. Fresh. 



FIG. XIII. The female reproductive organs, dissected as for Fig. XI. 

 The central cavity of the ovary has been laid bare on the left side. x 2. 

 Figs. XI. and XIII. were drawn from sexually mature specimens, and therefore represent 

 the organs as they appear during the breeding season. 



FIG. XIV. Two stages in the development of the ovisac. D. 3. 



A small portion of the germinal epithelium was selected, as described for that of the 

 Frog (Plate VI., Fig. XXV.), and placed on a slide in magenta solution. To avoid undue 

 pressure, a cover-slip was placed on either side of the preparation, upon which to support 

 that covering the object. Modified from Huxley (29). 



FIG. XV. Surface view of the Crayfish ovum at the close of segmentation (a stage 

 equal to that of the Frog, Fig. VIII., Plate VII.) 2 inch objective. 



FIG. XVI. Longitudinal section of a portion of the same, through a plane corre- 

 sponding with that of the reference line, bl, of Fig. XV. D. 2. 

 For further details see Eeichenbach (33). 



FIG. XVII. A surface view of the embryo at an early stage (the equivalent of the free- 

 swimming " Nauplius " stage of the typical Crustacean development). 



FIG. XVIII. Side view of an older embryo. After Kathke (32). 



