MEM. M. C. Z., IX. No. 3.— ACALEPHS, POLYPS. 39 
11. The same as fig. 10, seen in profile. The cesophagus (pharyngeal sac) descending from the mouth is already 
well formed, and the chambers to the number of four are also well limited. 
12. An embryo much larger than the preceding. The small compartment (a) is divided into three by the pair 
of partitions, 3. The compartment 8 of fig. 10 is also divided already into two (8 and 6) by the appear- 
ance of the partition, 4. Of the 8 septa thus acquired, the pairs numbered 1 and 2 are always more de- 
veloped than the others. 
Nore. — The order of development of the ‘‘septa” 2and 4 is claimed by the brothers Hertwig to 
have been interchanged, so that the true order of their appearance would be indicated by the numbers 
in parentheses. Compare explanations of figs. 31, 35, and 36. 
13. Profile view of the preceding, showing the mouth surrounded by 8 compartments with rounded bases. 
14. The same as in the two preceding figures, but the progress of the partitions being more considerable, the 
division into 8 compartments is more distinct ; the partitions, 1, are still much more advanced ; they 
already reach the central pad surrounding the mouth (peristome), and consequently begin to indicate the 
primitive division into halves. 
15. An embryo in which the 8 complete partitions have reached the peristome. 
16. Embryo with 8 divisions showing the partitions, 5, well advanced, and the beginning of the partitions, 6, in 
the lobe y. The two new chambers thus formed are respectively € and ¢. 
17. One of the various forms which the embryos assume when in motion. 
18. The same as fig. 17. It has been compressed a little to show the ‘‘septa” or mesenterial folds (7 mnt.). 
Only two of the iatter (1, 1) present the beginnings of the craspeda, or mesenterial filaments ; they are the 
primary folds. 7. 
19. An embryo represented with the form which it assumes when it swims rapidly. The tuft of cilia at the 
pedal pole is considerably elongated. 4. 
20. Profile view of an embryo. The mouth is supported on a snout-like prolongation, and the primitive cham- 
ber a/ has begun to send forth a tentacle. 1, Primary fold with mesenterial filament slightly developed. 42. 
21. Embryo further developed than the preceding, exhibiting already eight tubercles, which are the beginnings 
of the tentacles of the eight chambers first formed. 4. 
22. Oral aspect of a young actinia already approaching the form of the adult. The 12 tentacles of the first for- 
mation are already produced. The period of equalization in sixes, taken alternately, is in process of 
accomplishment and transforms the special embryonic form into one with regularly radial symmetry. The 
two cycles already begin to appear. The group of 7 lobes which has as its centre the tentacle a! is always 
distinguishable by the size of the latter, and by the development of the mesenterial folds,1. 3°. 
23. Young actinia with two well-pronounced cycles. The limits of the pedal disk are well marked, although the 
movements of the animal are still very tively, and the tuft of pedal cilia is very lone. 3,2. 
Young actinia already attached, viewed from the oral pole. In the intervals between the tentacles of the first 
eycle (a —¢ §—e (§—6?), 5—a!) there begin to be formed six new pairs of young tentacles, which in- 
crease the total number to twenty-four. 
Nore.— Of the three tentacles occupying each of the intervals (a — §, ¢—6, ete.) the middle one 
out-strips the other two, replacing (in size) y, e, 8, etc., and thus the middle ones come to constitute 
the second cycle. The third cycle is then composed of the remaining 12 (smallest) tentacles, which 
occupy the intervals between those of the first two cycles, thus regularly alternating with them. The 
subsequent increase in the number of chambers—and later, in that of the corresponding tentacles — 
is accomplished by the produetion of a pair of elements (mesenteries) in each of the 12 chambers above 
which are placed the smallest tentacles ; and by a process of substitution similar to that just described 
the middle ones of the three compartments (tentacles) thus formed comes to constitute the third cycle, 
while the two on either side of it become members of the fourth cycle. The fifth cycle is formed in a 
similar manner. ‘Since after the formation of each fresh eycle, the arrangement of the tentacles 
again becomes symmetrical (in sixes), it is obvious that all the equal-sized cycles except the first are 
formed of tentacles entirely heterogeneous as to age.” 
” 
to 
ha 
25. Face view — from the side of the seven-chambered group— of a young actinia with twenty-four tentacles, 
showing the relative development of the three pairs of mesenterial folds, 2, 4, and 5. 7%. 
26-31. Actinia (sp.?). From A. O. Kowalevsky, Observations on the development of the Ceelenterata. From 
the Publications of the Imperial Society of Friends of Natural Sciences, Anthropology, and Ethnography. 
Moscow: Katkov, 1873. 4to, 36 pp., 8 pl. (Russian.) Pl. IV. figs. 1-5, 7. 
26. The egg after its sexmentation. 
27. Infolding of the blastoderm. 
28. Invagination completed. 
29. Later stage in which the mouth-opening appears as a narrow slit and the first pair of mesenterial partitions 
has arisen. 
30. Radial section of the germ which is represented in fig. 29. 
