42 



EMBKYOLOGICAL MONOGRAPHS. 



PLATE XII. 



Development rif ZoAXTiiARi.v and Alcyoxakia. Figures from A. 0. Kowalevsky, Etiense 

 JouuDAN, WiLiucLM Buscii, Alicxaxdeh Agassiz, EdiVaud L. .Mark and Carl Claus. 



1-9. CcviantJiiis mcmbrnnaceus. l-fi, S, 9, from A. O. Kowalevsky, Ohserv.itions on tlie development of the 

 Ccelenterata. From the Publications of the Imiieri.al Society of Friends of Natural Sciences, Anthro- 

 pology, aud Etkiiography. Moscow: Katkov. 1873. (Uussian.) PI. VI. 



1. Egg after the segmentation is completed, 'p. The blastoderm, of a single layer of uniform columnar cells, 



embraces a comparatively large segmentation cavity. 



2. Stage showing the invagination of one half of the blastoderm into the other lialf. At the bottom of the 



cavity are .seeu fat globules {ijll. o!.) which came out through the iuvagiuatcd cells, en. 



3. A farther developed stage, already slightly elongated. 



4. Tlio edges surrounding the niouth-oiiening have been infolded to form the beginning of the stomach, jiju: 



NoTii. — This infolding alfccts principally only two opposing sides, whereby the cavity thus formed 

 is bounded on two siiles, a.s shown in tliis figure, by both layers {cc. and «!.)of the infcdded wall, whereas 

 at the two remaining sides the infolded jiortiou of the ectoderm is in innnediate contact with tlie superfi- 

 cial layer, the entoderm not being involved in the folding. The upper part of the cutodermic cavity is 

 thus divided into two lateral pockets. Compare fig. 9. 



5. .\ still more developed stage, in which four tentacles {/.a.) have a)ipeared. The rim or lip which surrounds 



.the mouth-opening is seen through the substance of a tentacle at lab. 



6. A swinnning larva of Cerianthus, whicdi shows, beside the four tentacles, a pair of oral tentacles {or. la.), a, 



pair of nu'senterial lilanients, and an indication of the so-ealleil anal jiore at the aboral end. 



7. Longitudinal section of a larva of Cc'rianthns niembranaceus. ^i". From Ftienne .lourdan, Ueehcrehes zoo- 



logiipies ct histologiriues sur les Zoanthaiies du Colfe dc Marseille. Ann. sci. nat., .ser. 6, zool., Tom. X., 

 Art. no. 1. PI. XVI. fig. 121. 



8. Transverse section through the posterior end of the larva represented in fig. 5. The longitudinal niuseleg 



of the wall, lying on the outer margin of the membrana propria, are .shown cut across at /jr. mii. Tliese 

 arc believed to have originated from the ectoderm. The cut ends of the longitudinal muscles on botli 

 sides tlij mesenteries are seen at !rj. mu'. 



9. Transverse section through tlic upper end of the same larva. 



10-1.'>. Cn-ijiiilliiisl {I)i<inlhra. vohilis I'.iiseh). From Wilhelm Busch, Deobachtungcn iilier Anatomic und Ent- 

 wickclung einiger wirbellosen Seethiere. Berlin : A. liirsehwald, 1851. Taf. XVll. figs. 1, 2, 4, 5, 7, 8. 



10. Young larva, uniformly ciliate ; too opaque to show interna! structure. 



