292 INVEETEBEATA CHAP. 



of P. coerulea, and the eggs of the British species can be artificially 

 fertilized and the larvae reared through their complete development 

 on a diet of the diatom Nitschia. 



According to Wilson, the eggs of Patella coerulea are found ripe 

 from March until June ; the eggs of the British species, on the 

 contrary, appear to ripen in October and November. The ripe eggs 

 are surrounded by a " chorion " ; this membrane, however, gradually 

 dissolves when the eggs are allowed to stand in sea- water. Artificial 

 fertilization is greatly assisted if both eggs and sperm, before being 

 brought together, are allowed to lie in sea-water which has been 

 rendered slightly alkaline by the addition of from 4 to 6 drops of 

 a 5 per cent solution of caustic soda to every 500 cc. of sea-water used. 

 In many cases more than one spermatozoon penetrates the egg and 

 abnormal development results, but these abnormally fertilized eggs 

 are recognizable from the fact that they divide at the first cleavage 

 into four blastomeres instead of into two only, as normally fertilized 

 eggs do. If, therefore, at the time of the first cleavage, the eggs 

 which have divided into two blastomeres only are picked out with 

 a fine pipette, a supply of normally fertilized eggs will be obtained 

 whose further development can be studied in detail. 



For his studies in cell-lineage Wilson preserved the cells simply 

 in acetic acid. To a watch-glass full of sea-water containing the eggs 

 a few drops of glacial acetic were added, and then, drop by drop, 

 dilute glycerine. After a short interval, when a sufficient quantity 

 of dilute glycerine had been added, strong glycerine was added in the 

 same way. The eggs were in this way slowly transferred to a medium 

 of thick glycerine, in which they were studied ; they thus became 

 absolutely transparent, whilst the cell boundaries showed up as dark 

 lines, so that the segmenting egg looked like a glass model. A slight 

 trace of stain with acid carmine is sometimes an improvement, but 

 in most cases Wilson considers its use superfluous. It must be 

 remembered that preparations made in this way are not permanent, 

 but they afford more insight into the cell -lineage than those 

 obtained by any other method. 



The egg divides into two and then into four completely equal 

 cells, and thus it is almost impossible to discriminate the quadrants 

 of the egg from one another. This is quite impossible when the four 

 first blastomeres meet quite evenly in the axis of the egg. But for 

 four semi-fluid masses to meet in a single vertical line of junction is 

 an impossibly unstable condition ; they either meet so as to leave a 

 vertical space between them, their inner angles being somewhat 

 rounded off, or else they meet so as to form a " cross furrow " at one 

 or both poles. By a cross furrow is meant the meeting of two of the 

 blastomeres along a short surface so as to separate the other two from 

 each other. If a given two, say B and D, meet thus at the vegetative 

 pole of the egg, then the other two, A and C, will similarly meet at 

 the animal pole of the egg, so that two short " cross furrows " at right 

 angles to each other are thus produced. 



