A CONTRIBUTION TO THE HISTORY OF THE 

 GERM-LAYERS IN CLEPSINE. 



C. O. WHITMAN. 



CONTENTS. 



INTRODUCTORY. 



I. Cleavage and the early estab- 

 lishment of Bilaterally Sym- 

 metrical Relations 1 08 



1. First Period, ending with the 



Eight-cell Stage 108 



2. Second Period, ending with 



the formation of Proliferat- 

 ing Blastomeres (Telo- 



blasts) Ill 



II. Origin of the Mesenteron .... 113 



1. Historical and comparative . 113 



Clepsine 113 



Nephelis 116 



Rhynchelmis (Euaxes) ... 124 

 Branchiobdella 130 



2. Observations 133 



Recapitulation 137 



III. The Ectoderm and its Products, 139 



1. The Ectoderm 139 



2. The Ventral Nerve-chain. . . 139 



Use of the term Germ- 

 bands . 140 



Germ-bands of the Head, 140 

 Germ-bands of the Trunk, 142 



PAGE 



Origin of the Ventral 



Nerve-chain .' 142 



(<z) Historical and Critical, 142 



(^) Observations 155 



3. The Larval Gland-cells .... 157 



4. Primary Sense-organs of the 



Lip 159 



5. The Nephridia 159 



6. The Pharyngeal Clefts 162 



IV. Special and General Questions, 162 



1. Larval Nephridia 162 



2. Significance of Nephric Cell- 



cords 165 



3. Questions relating to the 



Nephridia 166 



Original Function 166 



Original Basis 167 



4. The Origin of the Epider- 



mis 1 69 



5. Significance of the Telo- 



blasts 171 



6. Foetal and Larval Types of 



Development 172 



The origin and fate of the germ-layers in Clepsine have been 

 the subject of investigation and critical discussion by a number 

 of recent writers, who have reached conclusions so widely at 

 variance that one mightstillsay with Balfour, — " Our knowledge 

 of the development of the Discophora is in a very unsatisfac- 

 tory state." The origin of the mesenteron is very difficult to 

 trace, and ignorance of its history has led to the greatest con- 



