142 AN INTRODUCTION TO MODERN GENETICS 



AAA 



Fig. 67. Organ-Forming Substances in the egg of the Ascidian Styela. — The 

 animal pole A is uppermost, the vegetative pole v down. In A, just before fertiliza- 

 tion, there is a layer of yellow cytoplasm on the surface, and a clear area round 

 the female pronucleus in the upper part of the egg. The sperm enters near the 

 vegetative pole, and after fertilization (6) the yellow cytoplasm collects round 

 the sperm nucleus, with the clear cytoplasm from the female nucleus just above it. 

 In C the sperm nucleus moves up and meets the egg nucleus just below the equator 

 on one side, and the clear and yellow cytoplasms move up with it, making two 

 crescents (C and V) on this side, which will be ventral; the yellow cytoplasm 

 eventually forms muscles and mesenchyme, the clear forms ectoderm. Meanwhile, 

 the yolk tends to collect at the vegetative pole (YK), and above it on the dorsal 

 side appears a light grey area (G) which will form notochord and neural plate. 



(After Conklin.) 



Fig. 68. Cleavage and Shell Form in 

 Left- and Right-handed Gastropods. 



The left-handed (sinistral) form is on the 

 left, the right-handed (dextral) on the 

 right. The species shown is not Limnea. 

 (After Morgan, from Huxleyand de Beer.) 



case, the whole of the formation of the substances and their arrange- 

 ment into the definitive pattern occurs before the zygote nucleus is 

 formed. It is clear, therefore, that as regards one individual life-history. 



