A Cytological Study of Artificial Parthenogenesis etc. 151 



logical changes, but as soon as the egg is transferred hack into 

 normal sea water the process of membrane formation commences. 

 Small vesicles appear over the whole surface increasing in size until 

 they run together and form a clear transparent layer completely 

 surrounding the egg and bounded on the outside by the surface film 

 that has been raised up by this cytolytic process. The thickness of 

 this transparent outer layer is usually about 7 (.t but it may vary 

 considerably according to the amount of cytolysis that takes place 

 after the action of the butyric acid. 



The membrane formation is complete a few minutes after trans- 

 ference to normal sea water. It is accompanied by the more definite 

 appearance of a light space in the cytoplasm, marking the position 

 of the nucleus. The greater distinctness of this clear space at this 

 stage is due to the dissolution of the cytoplasmic granules immedi- 

 ately surrounding the nucleus to form a clear perinuclear zone. 



About 30 minutes after the completion of the subsequent treat- 

 ment with hypertonic solution, the cleavage aster appears. As it 

 develops the nucleus becomes still more distinct and the various stages 

 in the formation of the spindle and subsequent division of the nucleus 

 can be observed. In those eggs that segment normally the first three 

 cleavages give rise to eight cells of approximately equal size. The 

 fourth cleavage results in the formation of four smaller cells at one of 

 the poles. After this stage the divisions become less regular but eventu- 

 ally a blastula is produced which develops cilia on its outer surface 

 and becomes free swimming. These blastulae swim about at the sur- 

 face of the water and behave in exactly the same way as normal 

 larvae 1 ). The blastula undergoes the process of invagination giving 

 rise to a gastrula and from this is developed the pluteus with its 

 typical skeleton. 



The Unfertilized Ovum. 



In sections the unfertilized egg of S. purpuratus is rather opaque 

 owing to the large number of yolk granules scattered through the 

 cytoplasm. These granules are more abundant at the periphery, 

 thus forming a kind of ectoplasmic layer. The nucleus appears as 

 an oval, lightly-staining area, bounded by a definite membrane, and 



*) Those that develop from eggs treated simply with a hypertonic solution 

 (LOEB'S first method) are unable to rise to the surface of the water but can 

 only swim about the bottom. 



