THE MOSAIC STAGE OF DIFFERENTIATION 247 



presumptive eye-rudiments: a portion of these rudiments was 

 therefore left in situ, and another portion was included in the 

 rectangular piece. The portion left in situ developed into the eye- 

 cups in the normal position; the other portion after rotation de- 

 veloped into the eye-cups farther back. If the rectangular piece 

 was short, its hinder edge was in front of the ear-vesicles, and the 

 posterior pair of eyes then developed there. If on the other hand 

 the rotated piece was longer, so that its hinder edge was situated 

 behind the ear-vesicles, the posterior pair of eyes was behind the 

 ear-vesicles also^ (fig. 120). 



These results show in the first place that the eye-rudiments were 

 determined, since they could go on diflPerentiating normally in 

 abnormal surroundings. In some cases the four little eye-cups are 

 not equal in size, but the sum of the sizes of the left front and right 

 hind eye-cups is equal to that of the right front and left hind cups : 

 the two eye-cups formed from one original rudiment divided by 

 the cut are, together, equal to one normal eye. This means, there- 

 fore, that the eye-rudiment is not only qualitatively but also quanti- 

 tatively determined, and that its topographical limits are now fixed. 

 The actual time of onset of this determination cannot be stated, 

 since the rudiment continues to be in contact with the underlying 

 gut-roof, and progressive chemo-differentiation probably proceeds 

 after the experiment. It is to be noted that in the experiment on 

 ^ Spemann, 1912A. 



Fig. 120 

 Mosaic development and self-differentiation of the brain and eyes in Rmia 

 esculenta. A, Dorsal view of the brain of a normal larva. B, Similar view of a 

 larva in which at the neurula stage a square piece of neural plate with underlying 

 gut-roof was rotated through 180° ; the asterisks mark the line of junction between 

 the rotated and non-rotated regions. C, Similar larva in which the piece 

 rotated was longer. Note that the rotated regions of the brain have continued to 

 develop by self-differentiation : the mid-brain lies in front of the epiphysis or fore- 

 brain. There are four eyes, owing to the fact that the cuts went through the eye- 

 region and, after rotation, parts of the eye-region found themselves behind the 

 normal position (B) or, if the rotated piece was long, behind the ears. The sum 

 of the sizes of the four eyes is equal to that of two normal eyes. In spite of their 

 reduced size, the subdivided portions of the-eye- rudiments have become rounded 

 into cups approximating to the normal morphological differentiation. D, Neu- 

 rula, showing the operation. Av, auditory vesicle; Bb, between-brain ; Cp, 

 choroid plexus; Ep, epiphysis; Fb, fore-brain; Hb, hind-brain; Hg, habenular 

 ganglion ; Mb, mid-brain ; Mo, medulla oblongata ; O/, optic lobe ; Ov. eye-cup ; 

 Pc, posterior commissure. (From Spemann, Zool.Jahrb. Siippl. xv (3), 1912.) 



