Mosaic Development m Ascidian Eggs. 207 



This power of regeneration in the aduh is in striking contrast 

 with its lack in the egg and embryo and requires some explanation. 



It should not be overlooked that such injuries to the egg and 

 embryo as have been described in the preceding pages are prob- 

 ably more extensive and far-reaching than any which are capable 

 of being repaired in the adult. As Chabry says the destruction 

 of one of the first tw^o blastomeres is the same in its effect as the 

 destruction of the right or left half of the body of an adult. The 

 destruction of the anterior half of the egg is similar to the total loss 

 of the nervous system and notochord of the larva; while the death 

 of the posterior half corresponds to the destruction of the whole 

 of the muscular system and most of the mesenchyme of the larva, 

 since in each case the specific substance w^hich alone gives rise to 

 these organs is destroyed. Therefore these injuries are probably 

 much more extensive than any which have been practiced on the 

 adult animal. 



Furthermore, I am of the opinion that the extremely rapid 

 development of the ascidian egg and embryo may itself act as a 

 check on regulation. In Cynthia and Ciona the fully formed 

 larval stage is reached in about twelve hours after the fertilization 

 of the egg, and these larvae usually undergo metamorphosis into 

 the adult form within the next twelve hours. In Molgula the 

 development is even more rapid. It seems to me probable that 

 the restoration of the parts of the missing right or left half of a 

 larva might be fully accomplished if the larval life were longer. 

 In a right or left half larva one day old the ectoderm cells have 

 closed over the injured side, the notochord is complete, the neural 

 plate has invaginated, although abnormally, and the muscle cells 

 have begun to grow over from the uninjured to the injured side. 

 There is here evidence of considerable regulative ability and it 

 seems to me possible that, w4th more time before the metamor- 

 phosis, complete rows of muscle cells might be found on both 

 sides of the tail and that the mesenchmye cells might grow over 

 to the side on w^hich they are lacking and an atrial invagination 

 appear in them. 



Inasmuch as the only form of regulation shown by the ascidian 

 egg or embryo is this overgrowth of cells from the uninjured to the 

 injured side, it is probable that no amount of time would ever 

 suffice to produce an entire larva from the anterior or posterior 

 half of an egg or from a quarter or any smaller portion. As a 



