194 IN VEETEBRATA 



CHAP. 



At the next period of cleavage the endoclerm cells divide into 

 upper and lower daughters, and the generative cell divides into 

 right and left halves, and in this way a 62-cell stage is attained. 

 The two generative and four endodermic cells do not, however, 

 divide in the following period of cleavage, so that instead of 

 124 cells we have only 108 in the next stage. In this stage 

 the group of endoderrnic and generative cells is surrounded by a 

 horseshoe - shaped group of six cells, descendants of the similar 

 group in the 30-cell stage. This group constitutes the mesoderm 

 (Fig. 142, B). When the endodermal group becomes invaglnated, 

 as a result of inwardly directed cytotaxis, the rnesoderrn cells are also 

 invaginated (Fig. 142, 0). Kiihn derives the mesoderui from the 

 ectoderm because in the 30-cell and 62-cell stages the mother 

 cells of the mesoderm are cells which give rise also to daughters 

 which eventually form part of the ectoderm. But this comparison is 

 misleading and unjust. The wall of the blastula is differentiated into 

 regions, an eudodermic below, a mesodermic above this, and above this 

 again an ectodermic. The mere fact, that when the blastular wall 

 consists of few cells, mesodermic and ectodermic regions happen to 

 find themselves contiguous to one another so as to be within the 

 territory of one of the few nuclei, is of no importance. The rnesoderrn 

 of Polyphemus corresponds in position and origin to that of Astacus. 

 The early differentiation of the genital rudiment is a coninion feature 

 in animals of small size and short life-cycle. 



About the development of the other division of Pbyllopoda (i.e. 

 Branchiopoda) very little is known. The egg of Branchipus is stated 

 to undergo total segmentation, but the inner ends of the blastomeres 

 are said to coalesce into a yolky mass, on the surface of which is a 

 blastoderm. Doubt has recently been cast on this statement, and 

 there is no doubt that it requires reinvestigation. 



The development of Copepoda and Cirripedia seems to be funda- 

 mentally of a similar type. It may be regarded as a modification of 

 the type described for Astacus, a modification which is produced by 

 the diminution in absolute size of the egg, due to the smaller adult 

 size of the species, coupled with the fact that the nucleus and its 

 daughter nuclei are not diminished in the same proportion as is the 

 whole egg. Therefore the amount of nuclear matter relative to the 

 size of the egg is greater in these forms than in Decapoda, and the 

 nuclei are also far fewer in numbers. The result of this is to produce 

 a form of segmentation which might be variously described either as 

 holoblastic or meroblastic, according as one regarded the nucleus 

 which remains nearer the centre of the egg, as either (1) the nucleus 

 of a huge blastomere whose cell territory includes all the egg which is 

 not marked out into blastoderm, or (2) as a nucleus in unseginented 

 yolk which has not as yet had its cell protoplasm delimited. 



The development of Lepas and its allies has been studied by 

 Groom (1894), and the development of Lepas in its earlier stages has 

 been studied in great detail by Bigelow (1902). In this case the mother 



