18 Hjalmar Théel, 



1) That the preparation of the egg for the first segmentation requires 

 the longest time, and that in general the rapidity of segmenta- 

 tion is proportional to the size of the segments; thus the latest 

 segmentation stages take the shortest time. 



2) That at the first three cleavages the blastomeres are separated 

 almost simultaneously, but that afterwards a certain irregularity 

 occurs,* some sets of segments dividing earlier, others later. 



3) That the blastomeres of each of the three first segmentation 

 stages are almost equal, but that they afterwards become di- 

 stinctly unequal. 



4) That the difference in size is most evident in the fourth stage 

 of segmentation, when three quite different sizes are to be di- 

 stinguished. 



If we turn to the writers by whom the phenomena of segmen- 

 tation in the Echinoids have been carefully treated, we shall find that 

 a great conformity in this respect prevails in the three different orders 

 of Echinoids. Greatest attention has been paid, it is true, to the in- 

 ternal phenomena of segmentation, to the metamorphosis of the nucleus 

 etc., but notwithstanding there are a few accounts which treat the mu- 

 tual relation between the segmentation spheres and give support to the 

 supposition which I have mentioned above. Thus, for instance, Selenka*) 

 explains excellently the segmentation phenomena in Strongylocentrotus 

 lividus, Sphaerechinus granulans and Echinus microtuberculatus. In all 

 these forms the blastomeres are equal in each of the three first seg- 

 mentation stages, but unequal in the fourth stage, where three different 

 sizes, micromeres, »large» macromeres and »small» macromeres, are to 

 be found. However, some years before the same investigator ^) says 

 in connection with his researches on Echinus miliaris, Toxopneustes 

 brevispinosus, Strongylocentrotus lividus, Arbacia pustulosa and Echino- 

 cardium cordatum: »wie bei anderen Echinodermen, so werden auch bei 

 den Echiniden sehr bald Glrössendifferenzen zwischen den einzelnen Fur- 

 chungszellen wahrnehmbar, sobald nämlich die Zahl derselben über 16 

 oder 32 hinausgeht». Unfortunately, Selenka does not in this report 

 give any figures of the segmentation stages. However, in the paper first 



1) Die Keimblätter der Echinodermen. Studien über Entwickl. d. Thiere. II. 

 Wiesbaden 1883. p. 33—37. 



2) Keimblätter und Organanlage der Echiniden. — Zeitschr. f. wiss. Zool. Bd. 

 XXXni. 1879. p. 41—43. 



