11-21. Development of Isoropa. Figures from Anton Donen, Hernricn RataKe, and 
N. Boprerzky. 
11-18. Asellus aquaticus. From Dohrn, Die embryonale Entwicklung des Asel/us aquaticus. Zeitschr. wissensch. 
Zool., XVII., Taf. XIV., XV., 1867. 
11. Segmented egg. ¢, outer egg-membrane, chorion. According to Dohrn an inner membrane lies close upon 
the yolk. 
12. The blastoderm (57) now enyelopes the whole food-yolk. 
13. The blastoderm has become thickened on what will become the ventral side of the embryo. 
14. The inner egg-membrane ({) has separated from the yolk. ap, rudiments of the two first-formed appendages, 
referred by Dohrn (correctly 2) to the two pairs of maxilla. 
15. Embryo twelve hours later than fig. 14. 7, contour of the median portion of the body. , dorsal organ. 
Rudiments of the two pairs of antenne (I, II), mandibles (III), two pairs of maxille (IV, V), maxillipeds 
(VI), and the six anterior pairs of thoracic legs (VII-XII) have appeared. 
16. The embryo lies coiled within the egg, the dorsal flexure being so strong that the end of the abdomen and 
back of the head are in contact. The abdominal part of the body is now well developed and bears four 
pairs of appendages, the three anterior of which are rudiments of the gills, the fourth (XIX) being the ter- 
minal appendage of the adult. VII-XII, six pairs of thoracic legs. ju, appendage developed between the 
sixth pair of legs and the first pair of gills, later connected with sexual functions. The metastoma (71) is 
now present. 7, liver. a, anus. 
17. Later stage. The abdominal appendages are now bilobed. Previous to this stage the chorion has been cast 
off. The former inner membrane ({) now becomes the outer egg-membrane. The blastoderm at a stage 
much earlier than this has shed a cuticle (v) which now, as a larval membrane, forms a sac enveloping the 
embryo up to the time of hatching. 
18. Mouth parts of young before leaving the brood-sac of parent. 70, labrum. III, mandible. III’, mandibular 
palp. md, metastoma. IV, first maxilla. V, second maxilla. VWI, VI’, VI’, maxilliped. Within the 
transparent cuticle of the appendages are seen the setiferous appendages of the next stage. 
19. Young Ase/lus aquaticus before leaving the brood pouch of parent. The seventh thoracic somite and its 
appendages are not yet developed. Beneath the abdomen are seen three pairs of gills and the terminal 
appendages (XIX). From Rathke, Abhandlungen zur Bildungs- und Entwickelungs-Geschichte des Men- 
schen und der Thiere, Erster Theil, Taf. I. fig. 17, Leipzig, 1832. 
20, 21. Oniscus murarius. From Bobretzky, Zur Embryologie des Oniscus murarius, Zeitschy. wissensch. Zool., 
XXIV., Taf. XXII., 1874. 
20. Longitudinal section through embryo. sd, fore-gut, or stomodeum. pd, hind-gut, or proctodeum. ep, epi- 
blast. ms, mesoblast. hy, hypoblast cells. According to Bobretzky these cells originate from the blasto- 
derm and pass into and gradually absorb the food-yolk, increasing until they occupy the whole of the yolk 
space. 2, thickening of the epiblast along the median ventral line of the embryo from which is developed 
the nerve cord. x’, thickening of the epiblast which goes to form the brain. 9, cellular membrane lying 
within the inner egg-membrane. This membrane, which partially engirdles the embryo at this stage, origi- 
nates as a heap of thickened epiblastic cells on the dorsal side of embryo. These cells become attached to the 
inner egg-membrane (which seems to be made up of vitelline membrane and a larval skin together), spread 
and separate from the embryo excepting at one point where the connection persists by means of a short 
neck (X). This structure is homologous with the dorsal organ of Asellus, Moina, &e. 
21. Longitudinal section through an embryo at a much later stage. The invaginated portion of the epiblast which 
forms the stomodzeum is becoming differentiated into cesophagus and stomach. 7, rudiment of masticatory 
apparatus in the stomach. From the hypoblast cells have been formed the epithelial lining of the liver (7) 
and the mid-gut, or mesenteron. The mesenteron is not in communication with the stomach, and there is 
no line of demarkation between it and the epiblastic proctodeum. m, mouth. a, anus. %, intestine. 
1b, labrum. Below the mouth the section passes through the lower lip or metastoma. 4, heart, arising in 
the mesoblastie tissue. «, epithelium of intestine. « muscular outer coat of intestine, derived from the 
mesoblast. The outer part of the wall of the liver is also formed at the expense of the mesoblast, and below 
the stomach is seen a mass of mesoblast cells out of which is developed at a later stage the muscles which 
move the masticatory apparatus of the stomach. p, communication between liver and intestine. The 
nervous system (n, 2’) has become separated from the outer epiblast which now covers it in (c). 
