9. Larva after first ecdysis, about twelve hours after leaving the egg. Cyclops stage. The larva now has a 

 large dorsal shield and four free posterior segments. Eight pairs of appendages are present, two pairs of 

 antenna! (the second pair two-branched), mau<libles, ma.xilla;, two pairs of maxillipeds, and three pairs of 

 swimming-feet. The longer branch of the second antennae is furnished with a claw for fixing the larva. 

 The two pairs of maNillipeds (V, VI) are said by Claus to be develojied as two branches of one appendage, 

 representing the second niaxillie of the higher Crustacea. The three [jairs of swimming-feet are then 

 probably homologous with the three pairs of maxillipeds of Decapoda. The first free segment of the body car- 

 ries the second pair of swimming-feet, ocl, ocellus, y, glands lying on each side of the ej'e. 



10. Mouth-parts of the same stage, magnified 400 times. The mouth is situated at the end of a sort of proboscis 



formed by the prolongation of the labrum, I b, and the lower lip. The mandibles (III) are small, and the 

 maxilliB (IV) are small and furnished with a palp. 



11. Later stage of male larva, already parasitic on the perch. The posterior segmented part of the body has 



acquired a new segment at the expense of the anterior unsegniented portion. The swimming-feet have 

 disappeared as well as the spiral duct, although a remnant of the frontal papilla (a) persists. The outer 

 maxillipeds (V) have become united attheirends, whence a long rod (5) projects, which attaches the young 

 animal to its host, i, intestine, n, nerve, t, testis, v d, vas deferens, e, gland near the end of vas 

 deferens. 



12. Adult male, lateral view. The outer maxillipeds separate again. From v. Nordmann, Mikrographische 



Beitrage zur Naturgeschichte der wirbellosen Thiere, Zweites Heft, Taf. V. fig. 2, Berlin, 1832. 



13. Sexually mature female, seen from below. Natural length, 3°". The female is five times as long as the 



male. The outer maxillipeds remain fused and develop a sucking-disk, ovd, oviduct, f, tactile organ, 

 ij, cement gland (part of the female sexual apparatus, opening into the oviduct near the genital orifice). 

 &, receptacuhim seniinis. 



14-29. Cetochihis septcntrionalis, from Grobben, Die Entwicklungsgeschichte von Cctochilus scplcntrionalis 

 Goodsir. Arbeiten Zoolog. Inst. Univ. Wien, ML, Taf. XIX. -XXII., 1881. 



14. Egg before the first cleavage, in optical section. Natural size, .17"" diameter. The protoplasm and deuto- 



plasm are evenly distributed, and the egg is colorless, v m, vitelline membrane, a product of the yolk. 

 p c, second polar cell, the first usually being formed before the vitelline membrane, and hence escaping from 

 the egg. 7'«$, female pronucleus, p n^, male pronucleus. The exact origin of the polar cells was not 

 trai'ed in this case, but there seems little reason to doubt Grobben's interpretation of the structures as 

 above given. 



15. After the union of the male and female pronuclei, the resulting cleavage nucleus lies excentrically nearer the 



animal pole of the egg, as indicated by the polar cell. A total meridional cleavage is followed by an 

 equatorial cleft, and four deavage-sjiheres are formed, as shown in the figure, n c, nucleus of one of the 

 cleavage-spheres. At the core of the egg a small segmentation-cavity is already observable. The next 

 cleavage is meridional, and the resultant eight-celled stage passes by equatorial cleavage into a sixteen-celled 

 phase. The enclosed segmentation-cavity has now enlarged, and become the receptacle for the deutoplasm 

 ejected from the yolk. The polar cell now becomes involved with the cleavage products and pressed into 

 the interior of the egg. 



16. The next cleavage is in a plane perpendicular to the preceding, and thirty-two cells are thus formed. A 



period of rest of several hours' duration ensues before the cleavage is carried fnrtlier. At this stage a dilTer- 

 entiation appears among the constituent cells. In the two previous figures, the egg was seen from the side. 

 In this figure, the egg is turned so that the ventral side is toward the reader. It is now seen that one of 

 the ventrally situated cells has divided into two unequal parts, a small cell {a en) and a large one (c e n), 

 the other cells being arranged bilaterally with reference to them. The central larger cell (c e n) is prominent 

 on account of the greater amount of dcutoplasmio elements and coarsely granulated protoplasm. From 

 this cell are formed later the central portion of the hypoblast. From the subsequent development, it ap- 

 pears that the smaller cell (a e n), too, is a hypoblast cell. The four cells (I e n) lying on each side of the 

 larger and smaller cells, contain hypoblastic and epiblastic elements. The cell (m s) in the median line 

 behind the central hypoblast cell contains, besides epiblastic elements, all the elements of the mesoblast. 



1 7. Optical section of a later stage, lateral view. The polar cell {p r) has been pressed in between the cleavage 



cells into the cleavage cavity. All the cells take part in bounding this cavity, excepting the small anterior 

 hypoblast cell (n.en). In one of the cells at the upper left hand is seen a nuclear amphiaster preliminary 

 to the division of the cell. 



18. Later stage. The cleavage has now proceeded much further. The central hypoblast cell (ccn) is divided 



into two. The four lateral are in the process of division, and of the resulting cells those lying near the 

 central hypoblast cells and marked I en in the figure, together with the central (cen) and small anterior 

 cell (a e n), go to form the hypoblast, while the rest of the cells formed from the lateral cells belong to the 

 epiblast. From the division of the cell marked m s in Fig. 1 6, we now have four cells. The two larger, 

 anterior (;)») .•!), contain all the elements of the mesoblast, .and are called the primitive mesoblast cells. 

 The two smaller, posterior, are epiblast cells. Tlio anterior hypoblast cell (i e n) alone is undivided. AH 

 three genninal layers arc now formed, and .show a bilateral an-angement. In the next stage the central 

 hypoblast cells are divided by a transverse cleft into four. The primitive mesoblast cells have al.so divided, 

 so tliat there are four cells in this layer. 



