1892.] On the early Development of Cirripedia. 159 



while the egg is in the mantle cavity, whether the egg has been 

 fertilised or not. 



If fertilisation has not taken place, no further change ensues, and 

 the egg does not contract ; a second polar body is probably formed, 

 but owing to the resistance of the peri-vitelline membrane cannot 

 emerge, and is not seen. 



Tf fertilisation has taken place, the egg diminishes in size, and 

 commences to undergo rhythmical contractions, which cease only 

 when the protoplasmic and yolk portions are completely separated. 



The diminution in size is soon followed by the protrusion of clear 

 amoeboid processes at the anterior end of the egg, which are as often 

 withdrawn : from this amoeboid arises the second polar body, like 

 the first, by the division of the nucleus in the ordinary karyokinetic 

 manner. 



The protoplasm generally collects at the anterior (larger) pole, and 

 the yolk at the posterior (pointed) pole, in the well-known way. 

 The process does not represent a total division, as has been supposed, 

 into ectoderm and endoderm, but the formation of a teleolecithal egg, 

 the protoplasmic part of which will form the first blastomere, and now 

 rests upon a yolk portion, at first devoid of a special nucleus, but still 

 in communication with the protoplasmic half. 



The nucleus, at first small and peripherally situated at the anterior 

 pole (invisible without special preparation), becomes visible as a clear 

 spot or vesicle the segmentation nucleus occupying the centre of 

 the protoplasmic half of the egg. 



The nucleus divides, one daughter-nucleus remaining in the proto- 

 plasm, and the other passing into the yolk, the elements of which it 

 has the power of transforming into protoplasm ; this, together with 

 the bulk of any protoplasm left in the yolk, now emerges as a second 

 blastomere at the side of the first, which has in the meantime become 

 cut off from the yolk. 



The yolk becomes gradually covered by the successive emergence 

 of fresh cells, which process is accompanied by the division of the 

 cells cut off from it. The nucleus of these emerging protoplasmic 

 bodies or merocytes is given off either from a peripheral blastomere, 

 which has not yet been cut off from the yolk, or from a merocyte 

 which divides before emerging as a blastomere. The yolk may be 

 regarded as having the value of a single cell (macromere), which 

 gives off a succession of blastomeres (micromeres) much in the same 

 way as in the case of the epibolic eggs of Bonellia, where, however, 

 there are four macromeres, each of which behaves in the same way. 



The point where the blastoderm last covers the yolk represents 

 (except possibly in rare cases) the blastopore, the nucleus which 

 gives rise to both endoderm and mesoderm arising at or close to the 

 same spot. 



