11-21. Development of Isopoda. Figures from Anton Dohrn, Heinrich Rathke, and 



N. BOBRETZKY. 



11-18. Ascllus aqttalicus. From Dohrn, Die embrj'onale Entwicklimg AttsAseUus aquaticux. Zeitschr. wissensch. 

 Zool, XVII., Taf. XIV., XV., 1867. 



11. Segmented egg. e, outer egg-membrane, chorion. According to Dohrn an inner membrane lies close upon 



the yolk. 



12. The blastoderm (W) now envelopes tlie whole food-yolk. 



13. The blastoderm has become thickened on what will become the ventral .side of the embryo. 



14. The inner egg-membrane (f) has separated from the yolk, aj), rudiments of the two first-formed appendages, 



referred by Dohrn (correctly ?) to the two pairs of maxilla;. 



15. Embryo twelve hours later than fig. 14. ri, contour of the median portion of the body. X, dorsal organ. 



Rudiments of the two pairs of antenna; (I, ID, mandibles (III), two pairs of maxilhe (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 sti'ong that the end of the abdomen and 



back of the head are in contact. The abdominal part of the body is now well develojied and bears four 

 pairs of apjiendages, the three anterior of which are rudiments of the gills, the fourth (XI.K) being the ter- 

 minal ajipendage of the adult. VII-XII, six pairs of thoracic legs, y., appendage developed between the 

 sixth pair of legs and the first pair of gills, later connected with sexual functions. The metastoma {ml) is 

 now present. I, 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 (f) now becomes the outer egg-membrane. The blastoderm at a stage 

 much earlier than this has shed a cuticle (c) 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. Ih, labruni. Ill, niamlilde. Ill', mandibular 



palp, mi, metastoma. IV, first maxilla. V, second maxilla. VI, VI', VI", maxilliped. AVithin the 

 transparent cuticle of the appendages are seen the setiferous appendages of the next stage. 



19. Young Ascllus 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 Eathke, Abhandlungeu zur Bildungs- und Entwickelungs-Geschichte des Men- 

 scheu und der Thiere, Erster Thcil, Taf. I. fig. 17, I-eipzig, 1832. 



20. 21. Oiiiscus murarius. From Bobretzky, Zur Embryologie des Oniscus murarius, Zeitschr. wissensch. Zool., 



XXIV., Taf. XXII., 1874. 



20. Longitudinal section through embryo, sd, fore-giit, or stomodteum. 7)c?, hind-gut, or proctodeum. «p, epi- 



blast. m s, mesoblast. h y, hypoblast cells. According to Bobretzky these cells originate from the blasto- 

 derm,and pass into and gradually absorb the food-yolk, increasing until they occujiy the whole of the yolk 

 space, n, thickening of the epiblast along the median ventral line of the embryo fiom which is developed 

 the nerve cord, n', thickening of the epiblast which goes to fonn the brain. », cellular membrane lying 

 within the inner egg-membrane. This membrane, which partially engirdles the endiryo 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 bj' means of a short 

 ncrk (\). This structure is homologous with the dorsal organ o( Ascllus, Moina, &c. 



21. Longitudinal section through an embryo at a much later stage. The invaginated portion of the epiblast which 



forms the stomodsum is becoming differentiated into cesophagus and stomach, ir, rudiment of masticatory 

 apparatus in the stomach. From the hypoblast cells have been formed the epithelial lining of the liver (/) 

 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 proctodiBum. rn, month, a, anus, i, intestine. 

 lb, labrum. Below the mouth the section passes through the lower lip or metastoma. /(., heart, arising in 

 the mesoblastic tissue, k, ejiithclinm of intestine, i, 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 masticatoiy apparatus of the stomach, p, communication between liver and intestine. The 

 nervous .syst(!m (/;, n') has become separated from the outer epiblast which now covers it in (a). 



