108 GASTKOPODA. 



reduced in size (Fig. 40 H), or, as is most frequently the case, are 

 retained for some time longer as specially large cells (F and G), the 

 micromeres which have arisen from them shifting towards the animal 

 pole. The multiplication of the latter continues and leads to the 

 development of a cap of smaller cells which lies upon the macromeres 

 (/ T and G, etc.).* 



The great agreement of the various stages of cleavage in the Gastropoda 

 with those of the Turbellaria is very striking, as may be seen from a com- 

 parison of Fig. 40 with Fig. 75, Vol. i., p. 162. This is most marked in 

 stages C-E of Fig. 40, but the later stages F and G also show great re- 

 semblance to the corresponding stages in the Turbellaria (Vol. i., Fig. 75 F 

 and E). The radial structure does not, indeed, in the Gastropoda, extend as 

 far as in the Turbellaria, and cannot be directly compared with that of the 

 Polyclad embryo, because the radial cells of the latter are to be regarded as 

 the rudiment of the mesoderm (Vol. i., Fig. 75 C and E), while, in the 

 Gastropods, they are ectodermal. The radial structure in the two cases is, 

 however, only apparent, since the axes of the body can early be demonstrated 

 both in the Turbellaria and the Gastropoda. In a similar way we might 

 speak of a radial symmetry in connection with the stages of cleavage of many 

 other animals. . Further, this radial symmetry is very soon lost in the 

 Gastropod embryo by the appearance of the paired rudiment of the mesoderm 

 which gives the embryo a marked bilateral character. It has, however, been 

 asserted (Manfredi, No. 72), that in Aplysia, when the embryo consists of 

 eight blastomeres only, the mesoderm arises as four cells through division of 

 the four micromeres ; this, if true, considerably strengthens the resemblance 



* [Since this work was published, a large number of observations relating 

 to the early stages in the cleavage of the Gastropodan egg have been recorded, 

 all of which lend additional support to the belief expressed above (p. 106) that 

 a common type or plan could be recognised in the segmentation of the eggs of 

 all the various divisions of the Gastropoda. 



Thus, it is possible to trace the origin cf the ectoderm in every case to three 

 quartettes of micromeres which are cut off successively from the macromeres, 

 complication being introduced, in some cases, by the secondary division of the 

 first and second quartettes before the separation of the third and last quartette 

 from the macromeres, and in other cases, by the lesser development of the 

 yolk and consequent slighter differentiation of the blastomeres, thus making 

 it difficult to identify the macromeres. 



A very striking feature, common to the development of most Gastropodan 

 eggs and well shown in Fig. 40 C, I> and E, has been termed the spiral 

 cleavage. Thus, as early as the third cleavage, i.e., the formation of the first 

 quartette of micromeres, a curious obliquity becomes evident. This obliquity 

 is visible in the nuclear spindle even before the completion of the division, 

 but becomes more apparent at its close, when the cells of the upper quartette 

 (micromeres) lie in the furrows between the cells of the lower quartette 

 (macromeres). This "spiral" character is generally more apparent than is 

 represented in Fig. 40 C, but is well shown in D in the case of the second 

 quartette of micromeres. Spiral cleavage is of particular interest in view of 

 the fact that, in sinistral Gastropoda, the obliquity takes the reverse inclination 

 to that which is found in dextral forms (Crampton, No. V and Holmes, No. 

 XII I a i. For a general discussion of the significances of the forms of cleavage 

 in the Gastropodan egg see Conklin (No IV, pp. 185-192).— Ed.] 



