14 Hjalmar Théel, 



rich in deutoplasm, does not remain passive. On the contrary, we have 

 seen that the segments have gradually undergone important changes not 

 only in their mutual position but even in their external form. When 

 studying the phenomena of segmentation one gets the impression that 

 the segments alternately attract and repel each other, and that the 

 highest degree of attraction occurs when the nuclei after a completed 

 segmentation have obtained their rounded distinct form and are in a 

 state of repose. 



In all the eggs of Echinocyamus, of which I have studied the 

 segmentation, I always found the above phenomena repeated with re- 

 markable constancy not only during the first divisions but during the 

 whole cleavage process. 



The phenomena, which have been discussed above, are by no 

 means peculiar to the Echinocyamus, but are without doubt the rule 

 in the segmentation of other eggs. Thus, 0. Hertwig ') mentions some- 

 thing similar in Toxopneustes lividus (Lamk) and evidently Selenka ^) 

 has observed the same in several other Echinoderms. With regard to 

 other types of invertebrate animals, Loven ^) as early as in 1848 de- 

 scribes certain phenomena in the segmentation of the eggs of Mo- 

 diolaria marmorata Forb, which must doubtlessly be referred to the 

 same, and Mark *) points out that similar changes of the external form 

 and position of the segmentation spheres take place in Limax cam- 

 pestris Binney. 



At about one hour and 20 minutes after the fecundation the 

 second segmentation stage is accomplished by means of a second ver- 

 tical plane, forming right angles with the first cleavage plane. Here 

 also the constriction begins with a furrow at the animal pole of the 

 eggt PL I, fig. 15. We have now four perfectly equal cells, which 

 seen from above are almost round, PI. /, fig. 16^ but from the side 



1) Beiträge zui* Kenntniss der Bildung, Befruchtung und Theilung des thier- 

 ischen Eies. — Morpholog. Jahi-buch. I. Heft. 3. Leipzig 1875. p. 404. 



2) Die Keimblätter der Echinodermen. Studien über Entwickelungsgeschichte 

 der Thiere. Wiesbaden 1883. 



3) Beiträge zur Kenntniss der Entwicklung der Mollusca Acephala La- 

 mellibranchiata. (Aus d. Abhandl. d. k. Schwed. Akad. d. Wiss. 1848). Stock- 

 holm 1879. 



4) Maturation, Fecundation and Segmentation of Limax campestris, Binney — 

 Bull. Mus. Comp. Zool. at Harvard College. Vol. VI. N:o 12. Cambridge 1881. 

 p. 223—224. 



