kofoid: development of limax. 01 



In spite of the examination of a large number of eggs, and the repeated 

 killing of those whose age and approximate stage were known, I have not 

 been able as yet to obtain an egg showing the spindles resulting in this 

 division. Figures 38, 40, and 42 (Plate VI.) all represent stages sub- 

 sequent to the formation of M, and the other members of the (juartet 

 7.2. There is some evidence, however, as to the character of the divis- 

 ion. Figure 38 is a view of the vegetative pole of an egg of forty cells. 

 The four central cells have seven peripheral neighbors. Deeper focusing 

 reveals the presence of a large nucleus, lying within a definite cell 

 boundary. This is quite below the level of the nuclei of the vegetative 

 quartet. Its nucleus lies below and slightly nearer the median plane 

 than that of dJ-^. The superficial extent of this deeper lying cell is 

 limited to a narrow strip adjoining the cell with which it has arisen, 

 i. e. it is peripheral to cZ''^ The other members of the quartet 7.2 are 

 present, and when the test for the spiral is applied it is evident that 

 this is a left spiral, though the amount of the shifting is evidently not very 

 great. It is quite plain that in this case the cell d'''^, which gives rise to 

 the mesoderm, comes from d^-^ at the time of its division into d^-^ and 

 dJ-^. As in previous stages, the quartets d and b are in contact at the 

 ventral cross furrow. 



Figure 40 (Plate VI.) represents a forty-two-cell stage which has 

 recently been the scene of a number of divisions. Here, as in the egg 

 just discussed, the cell c?'''^ is very large, and is crowded in toward the 

 centre of the Q^^g, lying below d''-^ and slightly nearer the median plane. 

 It maintains a small crescentic connection with the exterior, between 

 (/^•^ and d^-^. The other members of the qiiartet 7.2 are present, and 

 show about the same nuclear conditions that d^-^ (= M) does. They 

 are therefore of about the same age. The divisions of the cells of the 

 quartet 6.1 in this e^g have evidently been very nearly equatorial, and 

 unless there should be at a subsequent period some shifting of the 

 mesoderm to a position nearer to the median plane, — as adopted in this 

 paper, — it would be necessary to orient this egg as Rabl and Blochmann 

 have oriented Planorbis and Neritina. Judging from the nuclear condi- 

 tions the division has taken place quite recently. Eggs of later stages 

 show that the mesoderm is generally placed bilaterally with reference 

 to the cross furrows and the prevailing quadrangular form of the egg 

 of those si ages. The division of this quartet is in all cases unequal, the 

 smaller cells lying at the vegetative pole. 



The conditions of the egg shown in Figure 42 (Plate VI.) were for a 

 long time very puzzling to me. It contains forty-five cells, which 



