PLATHELMINTHES 175 



flattened assumes the foi^m of a ffatworm. In Prosfomum 

 lineare an invagination of the ectoderm at the antei'ior end 

 of the body gives rise to the pliaryngeal sheath and the 

 phai'jnx.^ 



1 [The development of the Rhabdoccela and Acoela has recently been 

 studied by Pereyaslawzewa (see Appendix to Literature on Turbellaria, 

 Nos. I. and II.). 



According to this author, the development is the same in all the Acala 

 studied by her — Convoluta paradoxa, Aphanostoma diversicolor, Aph. 

 pulchella, and Darwinia variabilis — so that one description, that of Aph. 

 diversicolor, answers for all. The formation of jjolar cells and fecunda- 

 tion occur before the eggs are laid. The first cleavage results in two 

 cells of equal size. Preparatory to the second cleavage the nuclei elon- 

 gate, and approach the side opposite that where the polar cells have taken 

 refuge. With the second cleavage each of the two cells is divided into 

 two, one of which is four times as large as the other. The four cells 

 finally assume a symmetrical arrangement around the chief axis, the two 

 planes of cleavage becoming mutually iserpendicular. The two small cells 

 now divide, producing four of equal size arranged in a cross at the upper 

 pole. Then the two large cells divide into unequal parts, a larger basal 

 cell and a smaller one, lying nearer the plane of the four micromeres. 

 This eight-cell stage is a true blastula with cleavage cavity, and is quickly 

 followed by the division of the two basal cells, from which result four 

 basal cells of equal size (ten-cell stage), arranged not in a cross, but in a 

 row, which determines the long secondary axis. Two of the four basal 

 cells, the middle ones, are so crowded at their superficial ends by their 

 mates that they become wedge-shaped and finally forced into the cleavage 

 cavity. Thus gastrulation begins as a true emboly, but it is completed by 

 a process of ei^iboly. During invagination nearly all the pigment gra- 

 nules are accumulated in the lower ends of the four basal cells, which are 

 then abstricted as four small dark cells. Somewhat prior to this, how- 

 ever, the two cells lying a little below the plane of the four micromeres 

 divide, and their products are arranged symmetrically (on either side of 

 the plane determined by the chief axis and the long secondary axis). The 

 eight cells of the upper half of the egg now divide, producing sixteen. 

 With this (twenty-four-cell) stage gastrulation is well advanced, but it is 

 completed in the following stages by the overgrowth of the products of the 

 sixteen micromeres. "The two lateral cells, having given rise to the two 

 small (?) primitive-entoderm cells, represent the third layer : the meso- 

 derm." We understand the author to mean by " the two lateral cells " the 

 two cells which constitute the end of the row of four basal cells, but the 

 account is not satisfactory. A small but well-marked archenteron, com- 

 municating with the outside by means of a blastopore, exists from the stage 

 when the entoderm consists of only two cells, which assume a concavo- 



