156 H. W. BEAMS. 



MATERIAL AND METHOD. 



The material on which these observations were made consisted 

 of serial sections of Astcrias forbcsii eggs fixed in sublimate 

 acetic (2.5 per cent, acetic acid), sectioned at 5 /JL, and stained in 

 Heidenhain's heematoxylin. The eggs were procured, artificially 

 activated, and the cytological technique performed by Professor 

 C. E. Tharaldsen during the summers of 1921-22-23, at the 

 Marine Biological Laboratory at Woods Hole. The activating 

 agents used consisted of the single butyric acid method as de- 

 scribed by Lillie, and Loeb's double method. 2 The eggs had been 

 induced to mature uniformly by a special technique similar to that 

 described by Miss Brailey ('23). They were all activated at the 

 same time and specimens were then fixed at intervals of one minute 

 throughout the maturation from the breaking down of the germ- 

 inal vesicle to the first cleavage. The slides were studied in 

 chronological order. 



OBSERVATIONS. 



Normal Maturation. The primary oocyte of Astcrias forbcsii 

 ranges in diameter from IOO/A, to 125 /A; and the nucleus of such 

 eggs varies from one third to one half the diameter of the cell 

 (Fig. i). The nucleus is usually eccentrically located and the 

 nucleolus is found in the nucleus on the side nearest the periphery 

 of the egg. When activated during the breaking down of the ger- 

 minal vesicle, the nucleus shows a highly chromatic alveolar net- 

 work ; and within this network is a very delicate achromatic (linin) 

 reticulum. The nuclear wall begins to wrinkle and dissolve away 

 at a point nearest the periphery of the egg. The dissolution of the 

 nuclear membrane extends away from this point in all directions, 

 and becomes complete at the antipode on the median side of the 

 nucleus. The alveolar structure of the cytoplasm has itself be- 

 come finer. The loss of the nuclear membrane allows a slight 

 spread of the chromatin ; so that the mass seems to thin out pro- 

 gressively from the center ; although a quite definite demarcation 

 between chromatin and cytoplasm always remains. At this time a 

 small aster with its centrosome makes its appearance at a point just 



2 See Tharaldsen, " The origin and nature of cleavage centers in echino- 

 dcrm eggs." Jour. Exp. Zoo!., V. 44, no. I. 



