OCCUEEING IN THE NEIGHBOURHOOD OF PLYMOUTH. 33 



comes into contact with the blastoderm, which is here composed 

 only of epiblast; the cavity belongs, of course, to the segmentation 

 cavity. The question arises whether the periblast furnishes a 

 covering to the outer surface of the oil-globule as well as to the 

 deeper side. It probably does, but I cannot say with certainty. 



The time when the enclosure of the yolk is completed varies of 

 course with the temperature, but in my experiments it was effected 

 before the end of the second day. The differentiation of the organs 

 in the dorsal part of the embryo begins before it is complete, and 

 proceeds rapidly during the third day. Fig. 20 shows the stage 

 reached about the middle of the third day ; seven mesoblastic 

 somites are distinctly seen, the eyes are indicated, and. Kupffer^s 

 vesicle has appeared. On the fourth day (figs. 21, 22, 23) the 

 crystalline lens, the auditory vesicle, the heart, the intestine, and 

 the notochord are formed, and pigment appears in the skin. This 

 pigment is confined to the sides of the embryo, and to the deep 

 surface of the oil -globule ; there is none on the surface of the yolk ; 

 the pigment consists entirely of black dendritic chromatophores. 

 The development of these chromatophores in the periblast covering 

 the oil-globule on its deeper surface is another fact in connection 

 with the oil-globule which deserves special attention. I have 

 described the development of chromatophores in the periblast 

 covering the anterior part of the yolk in Pleuronectes mia-ocephalus, 

 and these connected with the oil-globule in the mackerel are in 

 homologous relations, but it is curious that they should be confined 

 to the surface of the oil-globule and absent from the rest of the 

 yolk. 



Fig. 24 shows the condition reached on the fifth day ; all the 

 organs are more developed, and the notochord is seen to be multi- 

 columnar. Pigment is still absent from the periblast covering the 

 surface of the yolk, except over the surface of the oil-globule. But 

 green chromatophores have appeared in addition to the black, and 

 are confined to two small groups on each side, one behind the eye, 

 the other at the base of the tail. The latter orgran has begfun to 

 grow out at the posterior end of the embryo. 



Some ova fertilized on June 29th, hatched on July 4th, the sixth 

 day, at a temperature of 14'5° C. I have not been able to give a 

 figure of the larva for rea&ons stated above, and can only give a few 

 notes of its characters. The distribution of the pigment is much 

 the same as in fig. 24 ; the notochord is multi-columnar, the mouth 

 not open, the anus is immediately behind the yolk. 



The special development of chromatophores round the oil-globule 

 is not a peculiarity of the mackerel, but seems to occur to a great 

 or less extent in all ova which contain oil-globules, especially if there 



VOL. II, NO. I. 3 



