KOFOID : DEVELOPMENT OF LIMAX. 45 



distn.nt, i. c. the first, being the lar<2;cr. In stained preparations the 

 larger one often contains a distinet nuclens with nnclear membrane and 

 chromatic grannies (Plate III. Figs. 20, 21). In the case figured here 

 the two globules are closely applied to the surface of the egg. In the 

 majority of instances, however, they lie at some distance from the egg in 

 the albumen, and in the living egg often seem to have no connection 

 whatever witli the vitelline surface. Thus it happens that the polar 

 globules are removed with the albumen in by far the larger part of the 

 eggs shelled. A phenomenon observed occasionally in the later stages of 

 the living egg is the increase in size of one of the polar globules and its 

 subsequent collapse (Plate I. Figs. 9-11). In one case the globule 

 reached a diameter half that of the egg itself. This is apparently 

 caused by the absorption of fluid from the albumen, and in the case 

 noted was followed by a collapse and a return to the normal size and 

 sliape. The eggs of Agriolimax agrestis vary a great deal in size, the 

 limits being from 80 ^ to IGO jU in diameter. The average size is about 

 100-110^. 



A. Orientation of the Embryo. 



In my treatment of the subject the orientation employed by "Wil- 

 son ('92), Conklin ('92) and Heymons ('93) is followed. Tlie first 

 cleavage plane is transverse, the second sagittal, in relation to the future 

 embryo. The polar globules are dorsal, the macromeres are ventral. 

 This does not, however, distinguish the anterior and posterior poles, and 

 I know of no way in which they can with certainty be determined in 

 Limax. The cells of the two ends are equal in size, generally, and when 

 slight difl'erences can be detected on careful measurement, it is impossible 

 to follow these differences during the protean phases of cleavage that 

 intervene between the two-cell stage and the appearance of the first 

 mesoderm cell, marking the posterior pole. Inasmuch as the mesoderm 

 cell {cP'^) comes from the left posterior quadrant, and is itself quite a 

 large cell, while its sister cell ((-V^) does not seem to be much smaller 

 than other members of its quartet, I have always placed the larger of the 

 two touching quadrants of the basal quartet in the position left posterior, 

 rather than right anterior. I have been compelled to orient arbitrarily 

 in many cases, when no difference in size could be detected, choosing 

 one of the two positions 180° apart. In choosing the larger cell I have 

 not followed the type of Umbrella, where without doubt the mesoderm 

 comes from the smaller of the two cells in contact at the ventral cross 

 fiu'row. 



