i SEGMENTATION 9 



After ;i resting period of about ;ui hour a seeond meridional 

 furrow develops in a manner simil;ir to the first and in a plane 

 perpendicular to the plane of the first furrow. This gradually 

 deepens and each hemisphere becomes divided into two blastomeres, 

 eaeh of \\hirli as In-fore assumes a spherical shape and then becomes 

 flattened out slightly against the other. Of the four blastomeres 

 \\hieh art- m\v present two, shown by subsequent development to 

 In- anterodorsal in position, are according to Cerfontaine normally 

 smaller than the other two. 



The two meridional furrows (a and /?) are followed after an 

 interval of about a quarter of an hour by a latitudinal furrow 

 slightly above the equator and this divides each of the four segments 

 into two. The egg now consists of eight blastomeres four smaller 

 micromeres on the apical side of the latitudinal division plane, and 



,** I I M 



Fi*;. 1. Apical view of Amphioxus i-ggs at the eiglit-blastomere stage. 

 (After E. B. Wilson, 1893.) 



A, <; Radial " type ; B, " Spiral " type ; and C, " Bilateral " type. 



four larger macromeres upon its abapical side. Each micromere 

 lies, according to Hatschek, exactly over the corresponding macromere 

 so that the apical side of the egg as seen from above looks like 

 A in Fig. 4. 



Wilson (1893), followed by Samassa (1898), has however drawn 

 attention to the fact that in a considerable proportion of cases the 

 cap of four micromeres is, as seen from above, rotated in a clockwise 

 direction through, an angle varying from to 45 (Fig. 4, B) thus 

 conforming to Wilson's " spiral " typa of segmentation or cleavage. 

 Again in a still smaller percentage of eggs at this stage the blasto- 

 meres are arranged according to Wilson's " bilateral " type (Fig. 4, C) 

 the eight blastoineres being arranged symmetrically on. each side of 

 the first division-plane but either two or all four macromeres being 

 displaced outwards somewhat from this plane. 



Fourth division. After another short interval (less than a 

 quarter of an hour) a new set of furrows appear bisecting each 

 of the already existing blastomeres so that the embryo comes to 

 consist of sixteen blastomeres arranged in two tiers, eight micromeres 

 above and eight macromeres below (Fig. 3, F). Hatschek described 

 this fourth set of furrows as being meridional (Fig. 3, F) while 

 according to Cerfontaine (1906) the division planes are when first 



