Miscellaneous, 481 



MISCELLANEOUS. 



The Emhrifonic Development of Comatula (Antedon rosacea). 

 By OsAVALu Seeliger, of Berlin. 



In tlie following paragraphs I commuDicate some of the results 

 of a detailed investigation the account of which wiU he puhlished in 

 Spengel's ' Zoologische Jahrhiicher,' but cannot appear forthwith on 

 account of the large number of plates. 



Segmentation is unequal. The segmentation cavity appears at 

 the stage with four blastomeres of equal size. The third and equa- 

 torial furrow differentiates four smaller cells, which are situated at 

 the animal pole, and four larger vegetative ones. The smaller 

 blastomeres next divide, and then the larger cells ; the stage with 

 sixteen cells is therefore preceded by one with twelve. Upon this 

 there nest appears round the animal pole a furrow rvmning parallel 

 to the equator, whereby sixteen equal-sized animal cells are formed, 

 Eo that a stage with twenty-four cells is reached. In consequence 

 of a mutual displacement of the small cells the segmentation cavity 

 closes up at the animal pole, while the eight large cells become 

 divided by an equatorial furrow into eight smaller and eight larger 

 cells, situated at the vegetative pole. It is not until the stage at 

 which forty-eight cells are present that the closure of the segmen- 

 tation cavity at the vegetative pole ensues and a typical blastula is 

 formed, the cells of which proceed to divide in such a way that 

 sixty-four, ninety-six, and one hundred and twenty-eight cells are 

 differentiated. 



In abnormal cases the equatorial furrow is not the third but the 

 first to appear, and segmentation commences with the formation of 

 two cells of unequal size. The smaller cell then divides, so that 

 the stage with two cells is followed by one with three. By the 

 extension of the second and meridional furrow to the larger cell 

 four blastomeres arise — two larger and two smaller ones. After 

 each of these has divided by a second meridional furrow into two 

 cells of equal size a stage with eight cells is produced, which then 

 entirely corresponds to the normal development. 



The gastrula arises by invagination at the vegetative pole in such 

 a manner that the chief axis of the ovum precisely coincides with 

 that of the embryo and of the subsequent larva. The mesenchyma 

 arises from the endoderm. 



It appears to me to be worth while mentioning an abnormal 

 bigastric form of embryo, of which I found one example among 

 normally developed blastulte. In the elliptical germ two gastrula 

 invaginations had developed and a number of mesenchyma cells had 

 appeared at their blind ends. This latter circumstance excludes the 

 suspicion that what was seen might possibly have been accidental 

 incurvations of tlie blastula wall, which would afterwards be evagi- 

 nated again : as is well known, this was the explanation given by 

 Metschnikoff of Fol's polygastral embryonic forms of Echinodcrms. 



As regards the further development I shall confine myself here to 

 treating of the nervous system. The free-swimming larva possesses 



Ann. & Mag. N. Hist. Ser. 6. VoL x. 33 



