THE STAEFISH SOLASTER ENDECA. 11 



Segmentation and Gastrulation (PI. I. figs. 1-6 ; PL III. figs. 24, 25). — In from 

 three to three and a half hours after they were shed the eggs began to separate 

 ofi" their vitelline membranes as "membranes of fertilisation"*. The process was 

 complete in from four to five hours. The width of the clear space left between 

 membrane and egg varied considerably, but was on an average equal to about a 

 seventh of the diameter of the egg itself. This membrane persists throughout 

 segmentation and until the blastula is beginning to rotate within it — that is, till 

 about the fifth day after fertilisation. 



Segmentation all through is total and equal, the first and second cleavage-planes 

 (PI. I. figs. 1-2) being vertical and at right angles to one another, while the third is 

 horizontal. The succeeding divisions follow with regularity and give rise to a solid 

 " morula " (figs. 3-4) made up of very numerous cells which are practically equal 

 in size. As many as three hundred such cells may be counted in a median section. 

 The flattening at the poles becomes less marked, but the mass floats as before, and the 

 under half is still paler in colour than the upper. It should be stated that while 

 equality of segmentation is certainly the normal condition, a slight degree of 



* Note on Cross-Fertilisation. — To the membrane of fertilisation is usually ascribed the function of keeping 

 out other sperms after the entrance of the one which is to fertilise the egg. The sperms crowd actively 

 round the ova immediately the latter are shed, and yet, as we have seen above, it was not till three hours after- 

 wards that the membrane even began to separate. This might seem to give ample time, were no other factors 

 at work, for the occurrence of polyspermy or even for the entrance of sperms of mixed origin. 



As is well known, cross-fertilisation may occur between widely different echinoderms. In this connection 

 a series of experiments I made some time ago affords the following results with reference to Solaster : — The 

 ova of Solaster endeca are strongly attractive to the spermatozoa of Solaster papposa and are fertilised by them. 



Segmentation, though often very unequal, was sometimes quite regular and led to a very considerable degree 

 of further development. Blastula, gastrula, and early larval stages were easy to obtain. Later, the larvee tended 

 to die off after showing various abnormalities of growth, such as malformation or loss of the larval arms and 

 vesiculation of parts of the larval body. Serial sections of the larvas showed very marked irregularity in the 

 internal cavities, the typical arrangement being interfered with in many ways, though there was evidently an 

 attempt at the formation of all the characteristic coeloms. The presence of several ciliated canals leading to 

 the surface from one or other of the internal cavities was a common deviation. Some of the hybrid larvje 

 lived for over two months, but none of them entered on the stage of sucker fixation which leads to 

 metamorphosis. 



The ova of Solaster papposa are also strongly attractive to the spermatozoa of S. endeca and are fertilised 

 by them. The resulting segmentation showed greater irregularity than in the converse experiment, but 

 unfortunately I had not the opportunity in this case of carrying the observations to later stages. 



In my experiments, the sperm of Solaster endeca was also tried with the ova of Echinus esculentw L. 

 Asterias rubens L., Uraster gladalis 0. F. M., Cribrella oculata Penn., Brissopsis lyrifera Forb., Eehino- 

 eardium cordatum Penn., and Synapta digitata Mont. The results as regards fertilisation were negative 

 in every instance, and the eggs of the sea-urchin alone exhibited the power of attracting the sperm. 



On the other hand, the ova of Solaster papposa attracted the sperm of Asterias ruhens, and underwent 

 irregular segmentation under their influence. The same ova neither attracted nor were fertilised by the sperm 

 of the other species mentioned above, 



c2 



