No. 3.] AMPHIOXUS AND THE MOSAIC THEORY. 583 



der Zellen durch eine meridionale Furche in zwei gleiche 

 Theile. . . . Man kann nach der Beschaffenheit und Anord- 

 nung der Zellen nur eine Hauptachse, die vom animalen zum 

 vegetativen Pole geht, unterscheiden." (10, p. 24.) This de- 

 scription applies to only a very small proportion of the embryos 

 examined by me. Two of these, which represent the nearest 

 approach to a true radial type I have seen, and agree closely 

 with the i6-celled stage of Antedon, are shown in Figs. 19-21. 



Even in these, however, the radial symmetry is not quite 

 perfect, on account of the bilaterality suggested by the slightly 

 smaller size of four of the lower cells (A~-D~). 



b. Bilateral form. This, which is much the most frequent 

 of all the forms, occurs in two distinct varieties, of which the 

 more usual is shown in Figs. 25 to 28. In this form each 

 of the four upper cells divides equally in a plane parallel to 

 that of the first cleavage. Each of the lower cells divides 

 unequally in an oblique plane, thus giving rise to an anterior 

 and a posterior pair of smaller cells (the secondary macro- 

 meres, A--B,~ C~-D~) lying at the respective ends of the 

 embryo, and to two corresponding pairs of larger cells (the 

 primary macromeres, A-B, C-D) that meet at the lower pole. 

 In typical cases the bilaterality is complete and is strikingly 

 apparent when the embryo is viewed from either pole. This 

 form of cleavage occurred in a large percentage of the embryos 

 examined, frequently modified, however, by variations in the 

 cleavage-planes of the upper four cells, a series of which are 

 shown in Figs. 22 to 24. In almost all cases an opening 

 (cleavage-pore) exists at the lower pole, which varies in size 

 from a mere pore (Fig. 27) to a very large opening (Fig. 30). 

 It gradually closes in later stages, but I have seen it persist 

 up to the gastrula stage (see p. 586). 



The second variety of the bilateral form (Figs. 13 to 18) 

 differs from the first in the fact that two of the secondary 

 macromeres (C,^ D~) are formed not at the end but at the 

 sides of the embryo (Fig. 1 5). This form closely approxi- 

 mates to the cleavage of the tunicate Clavelina, as may be 

 seen by a comparison of my Figs. 14, 15, with Figs. 7, 7,^ 7^ 

 of Van Beneden's and Juiin's paper (No. 31) {cf. Diagram, 



