398 ARTHUR T. MASTERMAN. 



be regarded as neither reliable nor conclusive. Of these 

 Kowalevski (9) held that the mesoblast arose by delamination 

 from the hypoblast^ and, so far as it goes, this agrees with the 

 present results. 



Metschnikoff (14) described and figured mesenchyme cells 

 present in the blastocoele cavity at an early stage before 

 gastrulation, and Foettinger (7) went even further, and claimed 

 to recognise the mesoblast cells at an even earlier stage. 

 Caldwell (3), in applying the method of sections to the ques- 

 tion, failed to corroborate these observations, and attempted 

 to explain Metschnikoff's figures on other grounds. He 

 failed to find any trace of mesoblast till well on in the process 

 of gastrulatiou. Roule (15), who followed him, also failed to 

 detect any mesenchyme cells until the pre-oral lobe and the 

 mouth were established. As stated above, there are no 

 figures, and Roule gives no account of his methods. Addi- 

 tional interest in the question has been roused by a recent 

 communication of Schultze (16), in which he reasserts the 

 origin of the mesoblast from mesenchyme cells. At the 

 present stage of the paper it will be sufficient to note that 

 he claims to find a mass of mesenchyme cells scattered 

 throughout the blastocoele cavity at a very early stage before 

 gastrulatiou. Again we are led to inquire, what were the 

 methods pursued ? Schultze gives three woodcut figures 

 which may or may not have been drawn from sections, and 

 this is all. 



Such being the case, special care was taken in this instance 

 to find out whether there are or are not mesenchyme cells 

 present in the early stage. The sections were stained very 

 deeply with eosin and hasmalum, and the nuclei were depicted 

 with remarkable clearness. In the great number of embryos 

 I have examined I have failed to find a single nucleus in the 

 blastocoele cavity till after gastrulatiou. The most one ever 

 finds are a few cytoplasmic strands and fragments, which are 

 stained with eosin alone, and show no trace of nuclei. In all 

 the stages up to the spherical blastula all the nuclei which 

 are present are arranged symmetrically in the ectoderm cells 



