June 24, 1920] 



NATURE 



517 



generally, but even in the adult it can always be 

 recognised by turning the feathers aside. Its position 

 suggests that it is in some way associated with the 

 pineal body, and dissection reveals a large pineal 

 gland directly beneath, though wholly cut off by the 

 skull. Early stages in the developing chick disclose a 

 yet more remarkable formation in the same region, 

 which leaves no doubt that the bare patch is really a 

 brow spot or pineal spot, the ostrich being the only 

 bird in which a permanent structure of this kind has 

 been described. 



Ostrich embryos of about twenty-six days' incuba- 

 tion — the full period being forty-two days — display a 



large pedunculate vesicle projecting from the middle 

 of the bare patch, surrounded by developing feathers 

 (Fig. i). The stalk is thick, deeply pigmented, and 

 rounded above, but irregular in outline below, while 

 the vesicle is thin-walled and almost transparent, the 

 whole structure strongly suggesting a stalked eye. 

 The outgrowth persists for a few days only, but all 

 ostrich chicks from about twenty to thirty days' in- 

 cubation reveal one stage or another in its develop- 

 ment or retrogression. 



Microscopic sections through the fully developed 

 organ reveal the condition represented in Fig. 2. The 



stalk is solid and broad below, with a thick epidermis 

 produced into small, irregular fimbriations and deeply 

 pigmented, while the vesicle is extremely thin-walled 

 and filled with a coagulable fluid. The underlying 

 dermal tissue is continued unchanged into the stalk, 

 but below it has undergone a peculiar sclerose modi- 

 fication. The whole structure is thus tegumental and 

 destitute of any special nervous or sensory elements. 

 Beneath it, but not represented in the figure, is the 

 well-developed pineal gland resting upon the pouch- 

 like dorsal sac. Before the vesicle is fully formed, 

 sections reveal that the basal part of the stalk is 

 double, as if two stalks have become fused, but only 

 NO. 2643, VOL. 105] 



one has elongated, and ends in the vesicle. At a late 

 stage in retrogression the two components are quite 

 distinct. 



A pedunculate vesicle, arising from the middle of 

 the head, renders the ostrich unique among birds as 

 well as among the entire vertebrate series. Its 

 general association, however, leaves no doubt that it 

 is a part of the pineal complex. The latter has, there- 

 fore, been investigated from the earliest chick stage, 

 and many significant features have been disclosed. 

 The primary pineal vesicle or epiphysis appears about 

 the third day of incubation as a simple, globular, 

 median outgrowth of the roof of the thalamen- 

 cephalon ; later, it forms a dense follicular system, 

 and communication with the third ventricle is lost. 

 At the time of its formation a small, solid upgrowth 

 takes place just anterior to it, and situated on the 

 right side, afterwards becoming tubular and detached, 

 and extending upwards and forwards over the left 

 cerebral hemisphere. All the evidence points to this as 

 a vestigeal parapineal organ, arising in close associa- 

 tion with the pineal organ, but detached from it. It 

 disappears about the tenth day of incubation. A para- 

 physial upgrowth arises in front of the velum trans- 

 versum, but persists for a few days only, and a dorsal 

 sac situated immediately under the pineal gland is 

 strongly developed. 



It is submitted that the vesicle is the embryonic, 

 persistent, integumental covering of a stalked parietal 

 eye which was present in the ancestors of the ostrich. 

 The sensory part of cerebral origin has degenerated, 

 as in all birds, but the protective corneal covering of 

 transparent epidermis and dermis which would 

 envelop it as it pushed its way upwards still reappears 

 in the embryo, remains for a very brief period, and 

 then retrogresses, the pineal patch being all that per- 

 sists in later life. The unique stalked character of the 

 eve is manife«:tlv a peculiarity to be correlated with 

 the presence of a covering of feathers in birds. 



J. E. DUERDEN. 



The Alligator Pear. 



The important notice of Persea gratissima in 

 Nature of May 27 may be usefully supplemented 

 from Madeira, where, during the last sixty years, from 

 ten or a dozen examples, the tree has become familiar 

 in every garden enclosure on the lower 500 ft. of our 

 mountain-sides— cultivated for its attractive form and 

 autumnal yield of valuable fruit. 



The revival of the short voyage from Madeira, three 

 days and a half, to Southampton has restored the 

 alligator pear to its former importance in the London 

 market, for no other locality can offer equal facility 

 for presenting this valuable esculent in condition to 

 satisfy the educated palate of those who know the 

 flavour of the fresh fruit. 



Grown from seed, P. gratissima begins to bear 

 fruit in from seven to ten years, attaining full 

 maturity in twenty years, when it has grown into a 

 spreading tree 30 ft. high or more, with dense light 

 green foliage, maturing an abundant crop in Septem- 

 ber and October. 



A single tree in full bearing will yield a market 

 value of from 8?. to 15Z., and the rich nutty-flavoured 

 fruit is in growing demand. 



In Madeira no serious efforts have been directed to 

 the improvement of the alligator pear, either by selec- 

 tion or grafting, and the large central seed still 

 remains as a reproach to us; but the stimulus of 

 increasing commercial value is at hand, and prefer- 

 ence will be given to increased food value. Some 

 years ago I suggested that the tree might be usefully 

 grafted on to Persea indica, one of the four Madeit^n 

 laurels, much hardier and with greater range, hoping 



