380 HAROLD H. PLOUGH. 



cell stage, by sucking them up into a very fine capillary pipette 

 with a bore about two thirds the diameter of the egg membrane. 

 If the pipette is of the proper size, and the eggs drawn up care- 

 fully by means of an attached rubber tube held in the mouth, 

 this method seldom injures the eggs. It was used throughout the 

 larger part of the work. When the eggs had passed into the two 

 cell stage, a small number were picked up with a slightly larger 

 pipette and placed in a few drops of sea water on a clean glass slide. 



The blastomeres were separated with fine glass needles used 

 free hand under a binocular microscope. It is possible to use 

 either one or two such needles, and with practice to acquire a high 

 degree of skill in their use. Needles for this work must be fine 

 enough to lie easily in the furrow between two cells, and stiff 

 enough to allow of a small amount of pressure. Ordinarily 

 needles which are sufficiently fine, work better if they are not too 

 long about ten times the diameter of the egg seemed to give 

 the most satisfactory results. This free hand method has been 

 used recently by a number of investigators for work in cutting 

 eggs in preference to the more rigid and much slower micro- 

 dissection apparatus. (Cf. especially Fry 1924, who has given a 

 complete account of the method of making the needles and their 

 use.) It has the advantage that many more eggs can be operated 

 on in a given period, and with practice the control is quite as 

 dependable. The needle is laid along the furrow between the 

 blastomeres, and drawn gently back and forth. Usually Arbacia 

 blastomeres can be separated cleanly and without injury by one 

 or two strokes. Often the egg sticks to the needle, and rolls with 

 it, but if this is not prolonged it seldom results in injury. In 

 some cases the blastomeres will separate the width of the needle, 

 but no further, remaining attached apparently by a thin bridge 

 of protoplasm. Such eggs were usually discarded, although the 

 blastomeres seldom come in contact again and eventually sep- 

 arate. 



The two cell stage of Arbacia with the membrane removed 

 exhibits certain differences from the normal which are of im- 

 portance in separating the blastomeres. Shortly after division 

 the two cells round out, torming two nearly perfect spheres having 

 a very narrow area of contact. They remain in this condition 



