ECHINOCYAMÜS PÜSILLUS. 9 



a fully even contour, being at the same time more and more removed 

 from the yolk by means of the rapid increase of plasma, which is a 

 result of the strong vital activity in the yolk. The intermediate space, 

 which becomes filled up with homogeneous plasma, finally attains very 

 large dimensions, reaching a thickness of 0,02 mm., while the yolk itself 

 has a diameter of about 0,088 mm. The membrane which defines this 

 space, is of course in its turn enveloped by the mucilaginous invest- 

 ment above mentioned. Probably the membrane, as it expands, increases 

 in firmness, thus preventing the entrance of any more spermatozoa. 



As is well known, it is supposed that the homogeneous plasma 

 which surrounds the yolk, is due to a certain contraction of the vitelline 

 plasma which effects simultaneously a delivery of plasmatic fluid. But 

 considering the great quantity of such fluid which would be delivered, 

 the volume of the yolk itself ought necessarily to be greatly diminished. 

 This is not the case however in Echinocyamus, or if it is so, it must 

 be in a very inconspicuous degree. If for instance the yolk before the 

 impregnation has a diameter of about 0,088 mm., this dimension remains 

 almost the same, though a layer of plasmatic fluid measuring up to 

 0,02 mm. in thickness is formed round it. I cannot give any other 

 reason for this fact, than that a rapid increase of new-formed plasma 

 takes place, caused by the activity of the impregnated egg. 



In normal eggs, when examining them with the highest power, I 

 have formed the impression that the yolk sends out very fine pseudopo- 

 dia-like processes through the plasma which fill up the intermediate space 

 between the yolk itself and the membrane. In other, probably abnor- 

 mal, eggs, I have seen, besides these fine processes, others of consi- 

 derable size and provided with plasmatic branches, so that one gets a 

 view in almost all respects resembling that figured by Selenka *) in his 

 description of the formation of the zona radiata. Though by no means 

 sure of it, I believe these latter eggs to be in some way imperfect, be- 

 cause they often during the course of their development shew morbid 

 symptoms, these large, branched pseudopodia-like processes being in my 

 opinion morbid. When studying the development of such eggs, I have 

 found that the surface of the yolk remains very uneven for a long 

 time and that often irregularities occur in the segmentation which lead 

 inevitably to the destruction of the embryo. 



1) Zoologische Studien. I. Befruchtung des Eies von Toxopneustes variegatus. 

 Mit drei Tafeln Leipzig 1878. 



Nova Acta Reg. Sog. Sc. Ups. Ser. III. 2 



