19. The mesoblast cells now retreat from the periphery of the egg into the cleavage cavity. The hypoblastic cells 
also sink in toward the centre, and the formation of a gastrula is thereby brought about. The figure shows 
the gastrula stage in horizontal optical section. 
20. Gastrula at a somewhat earlier stage, in longitudinal optical section. Owing to the small size of the cleavage 
cavity, and the large size of the hypoblast cells, the gastrulation is slow and difficult. When the (now 
eight) lateral and anterior hypoblast cells begin to sink, the four central hypoblast cells also sink a little, 
but as the former sink deeper, they compress the latter in such a way that they present the appearance 
shown in the figure. ¢c, epiblast. 
21. Gastrula seen from the side. The epiblast is seen in optical section. gm, gastrula mouth. 
22. Later stage. The gastrula mouth is closing up. Later it disappears entirely. That this side answers to the 
later ventral side is highly probable. 
23. Stage when the second pair of antennz, II (the first appendages to appear), have begun to appear. Optical 
cross-section. The other two pairs of nauplius appendages soon follow. The cesophagus of the nauplius 
arises from an invagination of the epiblast. 
24, Nauplius just hatched, seen from below. ‘he intestine (7) as yet has no anal opening. Under the bilaterally 
disposed ectoderm cells, at the posterior extremity of the body, are seen the primitive mesoblast cells (pms). 
From these are developed the mesodermie structures of the later-formed somites. The duration of the 
ege-development is about twenty-four hours. 
Norr.—In the nauplius of Cyclops serrulatus and Ergasilus Sieboldi, Grobben detected the presence of 
a dorsal organ equivalent to that in Phyllopoda, &c. See Arbeiten Zoolog. Inst. Wien, II, p. 262, Taf. 
XVI. figs. 61, 62, 1879. 
25. Later stage, lateral view. In the posterior part of the body, the rudiment of the genital system (ge) 
has already appeared. The intestine is now furnished with an anus (a7). 
26. A little older metanauplius larva, lateral view. The fourth pair of appendages (maxille) have appeared. 
n, brain. ’, secondary brain, a thickening of the ectoderm, interpreted by Grobben as a rudimentary 
organ representing the compound eyes of Phyllopoda and their ganglia, which do not develop further in the 
Copepoda. 
. Anterior portion of the same stage, seen from below. oes, esophagus. oc/, eye. 
8. A little older metanauplius than that represented by Fig. 26, from below. The anterior and posterior maxil- 
lipeds (V, VI) have now made their appearance, as well as the first pair of swimming-feet (VII). Under 
the cuticle is also seen the second pair of swimming-feet (VIII), which become free at the next moult. 
According to Claus and Grobben the so-called two pairs of maxillipeds of Copepoda really represent but one 
pair of appendages, the anterior pair being the outer branches, the posterior pair the inner branches. Both 
together will then represent the second pair of maxille of other Crustacea, and the first pair of swimming- 
feet will be the homologue of the first maxillipeds of Decapoda. Assuming this view to be correct, the 
reader must bear in mind that, in the figures of Copepoda on this plate, the Roman numerals higher than 
V must be lessened by one. 
29. First Cetochilus stage, from ventral side. The third pair of swimming-feet (IX) has appeared. mf, meta- 
stoma or paragnathite. At subsequent moults, new thoracic segments and appendages are developed, until 
the adult state is attained. 
wo pw 
a 
