UROCHORDA. 23 



is the same structure as the mass of elements which results in ordinary types 

 from the degeneration of the tail. If this suggestion is true it is difficult to 

 believe that this mass has any other than a nutritive function. 



The larva of Ascidia ampulloides described by P. van Beneden is 

 regarded by Kupffer as intermediate between the Molgula larva and the 

 normal type, in that the larval tail and notochord and a pigment spot are 

 first developed, while after the atrophy of these organs peculiar processes 

 like those of Molgula make their appearance. 



Sedentaria. The development of the fixed composite Ascidians is, so 

 far as we know, in the main similar to that of the simple Ascidians. The 

 larvae of Botryllus sometimes attain, while still in the free state, a higher 

 stage of development with reference to the number of gill slits, etc. than 

 that reached by the simple Ascidians, and in some instances (Botryllus 

 auratus Metscknikoff} eight conical processes are found springing in a ring- 

 like fashion around the trunk. The presence of these processes has led to 

 somewhat remarkable views about the morphology of the group ; in that 

 they were regarded by Kolliker, Sars, etc. as separate individuals, and it was 

 supposed that the product of each ovum was not a single individual, but a 

 whole system of individuals with a common cloaca. 



The researches of Metschnikoff (No. 32), Krohn (No. 25), and Giard 

 (No. 12), etc. demonstrate that this paradoxical view is untenable, and that 

 each ovum only gives rise to a single embryo, while the stellate systems are 

 subsequently formed by budding. 



Natantia. Our knowledge of the development of Pyrosoma 

 is mainly due to Huxley (No. 16) and Kowalevsky (No. 22). 

 In each individual of a colony of Pyrosoma only a single egg 

 comes to maturity at one time. This egg is contained in a 

 capsule formed of a structureless wall lined by a flattened epi- 

 thelioid layer. From this capsule a duct passes to the atrial 

 cavity, which, though called the oviduct, functions as an afferent 

 duct for the spermatozoa. 



The segmentation is meroblastic, and the germinal disc 

 adjoins the opening of the oviduct. The segmentation is very 

 similar to that which occurs in Teleostei, and at its close the 

 germinal disc has the form of a cap of cells, without a trace 

 of stratification or of a segmentation cavity, resting upon the 

 surface of the yolk, which forms the main mass of the ovum. 



After segmentation the blastoderm, as we may call the layer 

 of cells derived from the germinal disc, rapidly spreads over the 

 surface of the yolk, and becomes divided into two layers, the 

 epiblast and the hypoblast. At the same time it exhibits a 

 distinction into a central clearer and a peripheral more opaque 



