296 C. O. WHITMAN. 



(" Binnenholile ") arises between the lirst two blastomeres (Ano- 

 donta and Unio).i This space (p. 1^0) disappears during the 

 second division, but reappears after it is completed. The same 

 phenomenon is repeated in the subsequent divisions. "Die 

 Blasenform des Keimes, schon von der ersten Theiluug ange- 

 deutet . . . . ist bisher allgemein verkannt worden " (p. 163). 

 From the cases here cited, it is evident that the blastocoel begins 

 very early either as a cavity or a simple hole. The case of 

 Ciicullanus is that of Lumbricus and Paludina carried to the 

 extreme. Here there is no proper cavity — not even a fissure — 

 but the two cell-plates must nevertheless be regarded as a 

 Blastula, and not as a Morula. 



It is worthy of remark that the blastoccel, wherever it is pre- 

 sent, is at first bounded by a single cell-layer. The case of 

 Ascidia (Kowalevsky), of Sycandra (Schultz), of Anodouta and 

 Unio (Fiemming), of Clepsine and Euaxes, and numerous cases 

 like the latter, show that the blastoccel arises by the cells being- 

 pushed asunder in the process of cleavage. The case of Cucul- 

 lanus shows that the cleftvage may proceed in such a manner as 

 to avoid any such cavity. The fluid which collects in the hole or 

 cavity is the perivitelline liquid. 



(c) Mesoderm. 



The orgin of the mesoderm is a time-honoured problem, the 

 solution of which is still obscure. Some investigators derive it 

 from both ectoderm and entoderm ; others assert that it arises 

 independently of either. One claims that it owes its origin 

 to the ectoderm alone; and still another to the entoderm 

 plus certain migratory elements. Many believe that it arises 

 by delamination, a few by a process of infolding; others by 

 migration, and others still, by growths of one of the primary 

 lamellse into the blastocoel. In short, almost every possible 

 theory has been suggested, and advocated, at one time or another, 

 so that nothing remains but to test opinions already stated. 

 Whoever is now ambitious to launch a new theory, must approach 

 the subject from a phylogenetic stand-point ; here one can venture 

 without much danger, provided only he is content to set up one 

 of those theories which can iiever be proved or disproved. Of 

 such theories we have already at least two good examples. The 

 most plausible view of the genealogical origin of the mesoderm 

 is that first suggested by Lankester, Avhich will be considered 

 farther on. The question, whence comes the mesoderm, is by no 

 means easy to answer, as might be inferred from the fact that so 

 many different opinions exist in regard to it. In many cases, all 

 we know of its origin is, that it arises heticecn the two primary 

 lamellae just where these are continuous (Properistoma, Iliickel.) 

 As the edge of the blastopore is, so to speak, a neutral zone, how 

 ' I have observed the same in the egg of Planorbis. 



