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 
antenne (the second pair two-branched), mandibles, maxille, two pairs of maxillipeds, and three pairs of 
swimming-feet. The longer branch of the second antenne is furnished with a claw for fixing the larva. 
The two pairs of maxillipeds (V, VI) are said by Claus to be developed as two branches of one appendage, 
representing the second maxille of the higher Crustacea. The three pairs 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. ocJ, ocellus. +, glands lying on each side of the eye. 
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, 7, and the lower lip. The mandibles (II]) are small, and the 
maxilla (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 unsegmented 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 at their ends, whence a long rod (6) projects, which attaches the young 
animal to its host. 7, intestine. m, nerve. ¢, testis. wd, vas deferens. e, gland near the end of vas 
deferens. : 
12. Adult male, lateral view. The outer maxillipeds separate again. From y. 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. § tactile organ. 
nm, cement gland (part of the female sexual apparatus, opening into the oviduct near the genital orifice). 
&, receptaculum seminis. 
14-29.  Cetochilus septentrionalis, from Grobben, Die Entwickluagsgeschichte von Cetochilus septentrionalis 
Goodsir. Arbeiten Zoolog. Inst. Univ. Wien, III., 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. vm, vitelline membrane, a product of the yolk. 
pc, second polar cell, the first usually being formed before the vitelline membrane, and hence escaping from 
the egg. pngQ, female pronucleus. pg, male pronucleus. The exact origin of the polar cells was not 
traced 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 cleavage-spheres are formed, as shown in the figure. mc, 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 further. At this stage a differ- 
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 (ae x) and a large one (cen), 
the other cells being arranged bilaterally with reference to them. The central larger cell (ce) is prominent 
on account of the greater amount of deutoplasmic 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 en), too, is a hypoblast cell. The four cells (Ze) lying on each side of the 
larger and smaller cells, contain hypoblastic and epiblastic elements. The cell (ms) in the median line 
behind the central hypoblast cell contains, besides epiblastic elements, all the elements of the mesoblast. 
17. Optical section of a later stage, lateral view. The polar cell (yc) 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 (2en). 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 (ce) 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 Jem in the figure, together with the central (ce) and small anterior 
cell (aen), 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 msin Fig. 16, we now have four cells. The two larger, 
anterior (pms), contain all the elements of the mesoblast, and are called the primitive mesoblast cells. 
The two smaller, posterior, are epiblast cells. The anterior hypoblast cell (ae) alone is undivided. All 
three germinal layers are now formed, and show a bilateral arrangement. In the next stage the central 
hypoblast cells are divided by a transverse cleft into four. The primitive mesoblast cells have also divided, 
so that there are four cells in this layer. 
