CHAP, vii ANNELIDA 129 



embryology of this form has been worked out in great detail recently 

 by Woltereck (1902, 1903, 1905), and we select it as type for special 

 description. As, however, although Polyyordius occurs on both sides 

 of the Atlantic and in both North Sea and Mediterranean, it is not 

 very abundant or easy to obtain, some practical directions will be 

 given as to the means of dealing with the eggs of Pomatoceros, a very 

 common Polychaete belonging to the family Serpulidae. The develop- 

 ment of Pomatoceros, in the early stages at least, is almost identical 

 with that of Polygordius, and in one or two points even more primitive. 

 The eggs of all Annelida undergo cleavage of the spiral type, 

 which we have already studied in the case of the Platyhelminth 

 Planocera. In Annelida, as in Planocera, the ectoderm is separated 

 as three successive quartettes of micromeres. As in Planocera also, 

 a blastula consisting of relatively few cells is formed, which, by 

 invagination or epibole (see p. 92) is converted into a gastrula. 



METHODS 



Now for the study of such eggs the method of sections is of very 

 little use. This method requires that the egg to be studied should 

 consist of a large number of similar cells, so that a sample such 

 as a section presents would give a good idea of the whole ; but 

 where the egg consists of relatively few cells and these are in- 

 dividualized at an early stage of development the method obviously 

 fails. So there is nothing left but to make whole mounts and 

 endeavour (as Surface did in the case of Planocera) to identify and 

 trace the history of each individual blastomere. 



This procedure, as already mentioned (Chap. V. p. 10-i), is termed 

 the study of Cell-lineage ; it was introduced by the American 

 zoologist Whitman (1878), who first employed it in the study of the 

 eggs of Hirudinea, and it was taken up by a brilliant school, which 

 Whitman founded, one of the most prominent of which was Prof. 

 E. B. Wilson (1892). 



Prof. Wilson applied the method to the study of the development 

 of. the Polychaete Nereis, a work which threw much light on 

 Annelidan embryology. Other pieces of work of equal merit were 

 those of Tread well on Podarke (1901) and Child on Arenicola (1910). 



If, nevertheless, we select the work of a German for special 

 description, when the credit of most of the investigations belongs to 

 Americans, it is solely because the development of the type on which 

 he worked is so primitive and simple that, once it is known, all the 

 others can easily be described in terms of it. 



In order that the cells may be identified in whole mounts of eggs, 

 it is necessary that these should be rendered transparent, and that 

 they should be examined from all sides. As the eggs of many species 

 are opaque owing to the fact that they contain numerous yolk grains, 

 this is not easy to do. Prof. E. B. Wilson employs a mixture of 3 

 parts of glacial acetic acid and 1 part glycerine. This mixture 

 in many eggs dissolves the yolk granules and makes the whole of a 



VOL. I K 



