274 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
VIII. CONCLUSIONS. 
egg, are derived exclusively from the spermatazoün. 
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 tho vory 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 tho 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. 
T. The nervous system and the longitudinal musenlature 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 tho 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 belones to each of the two primary germ layers, Recent careful 
observations indicate that likewise in Amphioxus and the Vertebrates 
the mesoderm is derived from both primary germ layers. 
10. The longitudinal musculaturo 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. 
