274 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



VIII. CONCLUSIONS. 



1. In the maturation of the ascidian egg the polar globules arise at 

 the vegetative pole, i. e. in the future endodermal portion of the egg. 



2. The archoplasms (attraction spheres) of the first cleavage spindle, 

 and consequently of all subsequent spindles of the fertilized ascidian 

 egg, are derived exclusively from the spermatazobn. 



3. The archoplasm (attraction sphere) is not an organ of heredity, 

 since in sexual reproduction it is frequently derived from one parent 

 only. 



4. Cleavage in the ascidian egg is bilateral from the very beginning. 

 The course of cleavage is less variable in the egg of Ciona than in that 

 of Amphioxus or the Vertebrates, and is predetermined by the internal 

 constitution of the unsegmented egg. 



5. The first equatorial plane of cleavage does not separate completely 

 the two primary germ layers, though it does separate definitive endoderm 

 from definitive ectoderm. 



6. The fundaments of the principal organs are arranged in zones 

 around the chief axis of the egg. 



7. The nervous system and the longitudinal musculature of the larva 

 are derived from a common fundament, which is a (neuro-muscular) ring 

 of cells encircling the margin of the blastopore. This ring of cells must 

 be regarded as a part of the primary ectoderm. 



8. The chorda and mesenchyme (or trunk mesoderm) are derived from 

 another ring of cells lying just within the margin of the blastopore. 

 This ring of cells is to be regarded as a part of the primary endoderm. 



9. The mesoderm of Ascidians is therefore derived in part from the 

 primary ectoderm, and in part from the primary endoderm. It is 

 formed exclusively by cells of the two rings already mentioned, one of 

 which belongs to each of the two primary germ layers. Eecent careful 

 observations indicate that likewise in Amphioxus and the Vertebrates 

 the mesoderm is derived from both primary germ layers. 



10. The longitudinal musculature of the Ascidian tadpole is homolo- 

 gous with that of Amphioxus ; the mesenchyme of the Ascidian, with 

 the mesoderm lateral to the muscle plates in Amphioxus. 



11. The chorda should be regarded as a mesodermal organ. 



