No. 2. — On the Early Development of Limax} By C. A. Kofoid. 



Table of Contents. 



Page 



I. Introduction 35 



II. Metliods 37 



III. Noincncliit lire of Spiral (^leavaj^e 40 



IV. General Sketch of the Develop- 



ment 43 



V. Cleavage 44 



Introductory 44 



A. Urientation of the Embryo 45 



B. Discussion of Cleavage . 40 



C. Literature on Spiral Cleav- 



age 06 



Page 



D. The Mesoderm .... 75 



E. Theoretical Considerations 77 



F. The Cleavage Cavity . . 81 



1. In Limax 81 



2. Literature 91 



3. Experimental .... 104 

 VI. Blastopore and Gastrulation . lOG 



Addendum HI 



Literature Cited 112 



Explanation of Plates 118 



I. INTRODUCTION. 



The question of the origin and liistory of the mesoderm, and its rela- 

 tion to the body cavity in the Mollusca, is one of prime interest and 

 importance. The employment of the mollusk as the type of the 

 " Pseudocoels " by the Hertwigs ('81) in their " Coelomtheorie " was 

 founded on the non-existence in mollusks of a true body cavity, the 

 mesencbymatous nature of the musculature, and the origin of the ner- 

 vous system, in part at least, from the mesoderm ; in a word, on the 

 nature of tbe middle germ layer in its origin and later history. Since 

 the puiilication of this important work many additions have been made 

 to our knowledge of the Mollusca. There is a notable agreement among 

 later investigators, especially Schmidt ('90), Miss Henchman ('91), and 

 Erlanger ('91), as to the ectodermal origin of the nervous system 

 in this group. Studies in comparative anatomy, particularly of that 

 primitive group, the Solenogastres, have led to the general acceptance 

 of the view that the pericardium of the Mollusca is the homologue 

 of the ccelom of the " Enterocoels " of the Hertwigs. This view is 

 based upon the relationship of the pericardium to both the se.xual 

 and excretory systems, embryology however having lent little support 



1 Contributions from the Zoological Laboratory of the Museum of Comparative 

 Zoology at Harvard College, under the direction of E. L. Mark, No. XLVIII. 

 VOL. xxvii. — NO. 2. 3 



