CLEAVAGE AND DIFFERENTIATION 



125 



Fig. 59 



D 



Mosaic development and prelocalisation in the egg of Ascidians (Styela). A, B, 

 Normal development, dorsal surface views : A, Late gastrula. The neural plate 

 (n.p.) overlies the notochord rudiment (not seen) ; and the muscle-rudiments (tns.) 

 border the blastopore laterally. B, Neurula. The neural tube (n.t.) has formed, 

 and the mesenchyme (m'ch.) is visible. C, D, Sections of abnormal neurula 

 stages derived from eggs centrifuged before the first cleavage. The disarrange- 

 ment of organ-forming substances by centrifuging had led to the disarrangement 

 of organ-rudiments. Endoderm (end.) and notochord (ch.) appear on the outside, 

 ectoderm (ect.) and neural plate substance (-9ts.) on the inside. Eye spots (E.) and 

 muscle-rudiments are also ectopic. (From Conklin, Chap, ix of Cowdry, General 

 Cytology, Chicago, 1924.) 



